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/lib/themes.rb

@@ -63,7 +63,8 @@ class Themes
   end
 
   def skins_for(name)
-    @flavours[name]['skins']
+    skins = @flavours[name]['skins']
+    skins.include?('default') && skins.include?('mastodon-light') ? ['system'] + skins : skins
   end
 
   def flavours_and_skins