Merge commit '02ea161506' 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.

config/settings.yml

@@ -17,7 +17,7 @@ defaults: &defaults
   preview_sensitive_media: false
   noindex: false
   flavour: 'glitch'
-  skin: 'default'
+  skin: 'system'
   trends: true
   trends_as_landing_page: true
   trendable_by_default: false