viewer: migrate to vue 3, general refactoring and cleanup

Non-exhaustive list of fixes and improvements done at the same time:

- html default background to grey (avoids white flash during init)
- unified links behavior
- added more theme variables
- removed the flex-expand transition (it wasn't working) and replaced it
  with a slide
- fixed LdLoading not centered on the content
- title on removable tags
- fixed an issue with encoded URI from vue-router
- unified Item resource URLs
- removed the iframe for PlainTextViewer (it wasn't working properly)
  and replaced it with a pre
- fixed clear and search buttons tabindex
- fixed the information panel bumping up during the fade animation of
  tag's dropdown
- fixed some focus outlines not appearing correctly
- moved CSS variables to the :root context
- Code cleaning

GitHub: closes #217
GitHub: closes #300
GitHub: closes #297
GitHub: closes #105
GitHub: closes #267
GitHub: closes #275
GitHub: closes #228
GitHub: closes #215
GitHub: closes #112

7 files changed, 98 insertions, 281 deletions

diff --git a/viewer/src/assets/scss/_buefy_variables.scss b/viewer/src/assets/scss/_buefy_variables.scss
deleted file mode 100644
index e008269..0000000
--- a/viewer/src/assets/scss/_buefy_variables.scss
+++ /dev/null
@@ -1,171 +0,0 @@
-/* ldgallery - A static generator which turns a collection of tagged
---             pictures into a searchable web gallery.
---
--- Copyright (C) 2019-2020  Guillaume FOUET
---
--- This program is free software: you can redistribute it and/or modify
--- it under the terms of the GNU Affero General Public License as
--- published by the Free Software Foundation, either version 3 of the
--- License, or (at your option) any later version.
---
--- This program is distributed in the hope that it will be useful,
--- but WITHOUT ANY WARRANTY; without even the implied warranty of
--- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
--- GNU Affero General Public License for more details.
---
--- You should have received a copy of the GNU Affero General Public License
--- along with this program.  If not, see <https://www.gnu.org/licenses/>.
-*/
-
-// Included below are all the defined variables from Bulma
-// Modify as needed, removing the !default attribute.
-
-// Colors
-
-$black: hsl(0, 0%, 4%) !default;
-$black-bis: hsl(0, 0%, 7%) !default;
-$black-ter: hsl(0, 0%, 14%) !default;
-
-$grey-darker: hsl(0, 0%, 21%) !default;
-$grey-dark: hsl(0, 0%, 29%) !default;
-$grey: hsl(0, 0%, 48%) !default;
-$grey-light: hsl(0, 0%, 71%) !default;
-$grey-lighter: hsl(0, 0%, 86%) !default;
-
-$white-ter: hsl(0, 0%, 96%) !default;
-$white-bis: hsl(0, 0%, 98%) !default;
-$white: hsl(0, 0%, 100%) !default;
-
-$orange: hsl(14, 100%, 53%) !default;
-$yellow: hsl(48, 100%, 67%) !default;
-$green: hsl(141, 71%, 48%) !default;
-$turquoise: hsl(171, 100%, 41%) !default;
-$cyan: hsl(204, 86%, 53%) !default;
-$blue: hsl(217, 71%, 53%) !default;
-$purple: hsl(271, 100%, 71%) !default;
-$red: hsl(348, 100%, 61%) !default;
-
-// Typography
-
-$family-sans-serif: BlinkMacSystemFont, -apple-system, "Segoe UI", "Roboto", "Oxygen", "Ubuntu", "Cantarell",
-  "Fira Sans", "Droid Sans", "Helvetica Neue", "Helvetica", "Arial", sans-serif !default;
-$family-monospace: monospace !default;
-$render-mode: optimizeLegibility !default;
-
-$size-1: 3rem !default;
-$size-2: 2.5rem !default;
-$size-3: 2rem !default;
-$size-4: 1.5rem !default;
-$size-5: 1.25rem !default;
-$size-6: 1rem !default;
-$size-7: 0.75rem !default;
-
-$weight-light: 300 !default;
-$weight-normal: 400 !default;
-$weight-medium: 500 !default;
-$weight-semibold: 600 !default;
-$weight-bold: 700 !default;
-
-// Responsiveness
-
-// The container horizontal gap, which acts as the offset for breakpoints
-$gap: 32px !default;
-// 960, 1152, and 1344 have been chosen because they are divisible by both 12 and 16
-$tablet: 769px !default;
-// 960px container + 4rem
-$desktop: 960px + (2 * $gap) !default;
-// 1152px container + 4rem
-$widescreen: 1152px + (2 * $gap) !default;
-// 1344px container + 4rem;
-$fullhd: 1344px + (2 * $gap) !default;
-
-// Miscellaneous
-
-$easing: ease-out !default;
-$radius-small: 2px !default;
-$radius: 3px !default;
-$radius-large: 5px !default;
-$radius-rounded: 290486px !default;
-$speed: 86ms !default;
-
-// Flags
-
-$variable-columns: true !default;
-
-// The default Bulma derived variables are declared below
-
-$primary: $turquoise !default;
-
-$info: $cyan !default;
-$success: $green !default;
-$warning: $yellow !default;
-$danger: $red !default;
-
-$light: $white-ter !default;
-$dark: $grey-darker !default;
-
-// Invert colors
-
-$orange-invert: findColorInvert($orange) !default;
-$yellow-invert: findColorInvert($yellow) !default;
-$green-invert: findColorInvert($green) !default;
-$turquoise-invert: findColorInvert($turquoise) !default;
-$cyan-invert: findColorInvert($cyan) !default;
-$blue-invert: findColorInvert($blue) !default;
-$purple-invert: findColorInvert($purple) !default;
-$red-invert: findColorInvert($red) !default;
-
-$primary-invert: $turquoise-invert !default;
-$info-invert: $cyan-invert !default;
-$success-invert: $green-invert !default;
-$warning-invert: $yellow-invert !default;
-$danger-invert: $red-invert !default;
-$light-invert: $dark !default;
-$dark-invert: $light !default;
-
-// General colors
-
-$background: $white-ter !default;
-
-$border: $grey-lighter !default;
-$border-hover: $grey-light !default;
-
-// Text colors
-
-$text: $grey-dark !default;
-$text-invert: findColorInvert($text) !default;
-$text-light: $grey !default;
-$text-strong: $grey-darker !default;
-
-// Code colors
-
-$code: $red !default;
-$code-background: $background !default;
-
-$pre: $text !default;
-$pre-background: $background !default;
-
-// Link colors
-
-$link: $blue !default;
-$link-invert: $blue-invert !default;
-$link-visited: $purple !default;
-
-$link-hover: $grey-darker !default;
-$link-hover-border: $grey-light !default;
-
-$link-focus: $grey-darker !default;
-$link-focus-border: $blue !default;
-
-$link-active: $grey-darker !default;
-$link-active-border: $grey-dark !default;
-
-// Typography
-
-$family-primary: $family-sans-serif !default;
-$family-code: $family-monospace !default;
-
-$size-small: $size-7 !default;
-$size-normal: $size-6 !default;
-$size-medium: $size-5 !default;
-$size-large: $size-4 !default;
diff --git a/viewer/src/assets/scss/buefy.scss b/viewer/src/assets/scss/buefy.scss
deleted file mode 100644
index 2bc4985..0000000
--- a/viewer/src/assets/scss/buefy.scss
+++ /dev/null
@@ -1,47 +0,0 @@
-/* ldgallery - A static generator which turns a collection of tagged
---             pictures into a searchable web gallery.
---
--- Copyright (C) 2019-2020  Guillaume FOUET
---
--- This program is free software: you can redistribute it and/or modify
--- it under the terms of the GNU Affero General Public License as
--- published by the Free Software Foundation, either version 3 of the
--- License, or (at your option) any later version.
---
--- This program is distributed in the hope that it will be useful,
--- but WITHOUT ANY WARRANTY; without even the implied warranty of
--- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
--- GNU Affero General Public License for more details.
---
--- You should have received a copy of the GNU Affero General Public License
--- along with this program.  If not, see <https://www.gnu.org/licenses/>.
-*/
-
-@import "~bulma/sass/utilities/initial-variables";
-@import "~bulma/sass/utilities/functions";
-// 1. Set your own initial variables and derived
-//    variables in _variables.scss
-@import "buefy_variables";
-
-// 2. Setup your Custom Colors
-@import "@/assets/scss/theme.scss";
-
-@import "~bulma/sass/utilities/derived-variables";
-
-// 3. Add new color variables to the color map.
-$addColors: (
-  "green": (
-    $green,
-    $green-invert,
-  ),
-  "purple": (
-    $purple,
-    $purple-invert,