Merge commit '02ea16150647ac3baf0bb8a89203ccc7200b4a2f' into glitch-soc/merge-upstream

Conflicts:
- `app/lib/themes.rb`
- `app/views/layouts/application.html.haml`
- `app/views/layouts/embedded.html.haml`
- `app/views/layouts/error.html.haml`
- `config/settings.yml`

All these conflicts are because glitch-soc and upstream have different theming
systems and upstream changed a few things to have dynamic theme selection based
on system settings.

Conflicts were solved to take that into account, and `current_theme` has been
changed in the process to return a tuple of `[flavour, skin]` to be used in
the `theme_style_tags` helper.

app/controllers/concerns/theming_concern.rb

@@ -12,13 +12,13 @@ module ThemingConcern
   def current_skin
     @current_skin ||= begin
       skins = Themes.instance.skins_for(current_flavour)
-      [current_user&.setting_skin, Setting.skin, 'default'].find { |skin| skins.include?(skin) }
+      [current_user&.setting_skin, Setting.skin, 'system', 'default'].find { |skin| skins.include?(skin) }
     end
   end
 
   def current_theme
     # NOTE: this is slightly different from upstream, as it's a derived value used
     # for the sole purpose of pointing to the appropriate stylesheet pack
-    "skins/#{current_flavour}/#{current_skin}"
+    [current_flavour, current_skin]
   end
 end