Merge branch 'master' into glitch-soc/merge-upstream

Conflicts:
- `Gemfile.lock`:
  Not a real conflict, just a glitch-soc-only dependency too close to a
  dependency that got updated upstream. Updated as well.
- `app/models/status.rb`:
  Not a real conflict, just a change too close to glitch-soc-changed code
  for optionally showing boosts in public timelines.
  Applied upstream changes.
- `app/views/layouts/application.html.haml`:
  Upstream a new, static CSS file, conflict due to glitch-soc's theming
  system, include the file regardless of the theme.
- `config/initializers/content_security_policy.rb`:
  Upstream dropped 'unsafe-inline' from the 'style-src' directive, but
  both files are very different. Removed 'unsafe-inline' as well.

app/views/layouts/application.html.haml

@@ -28,6 +28,8 @@
         = javascript_pack_tag "locales/#{@theme[:flavour]}/en", integrity: true, crossorigin: 'anonymous'
     = csrf_meta_tags
 
+    = stylesheet_link_tag '/inert.css', skip_pipeline: true, media: 'all', id: 'inert-style'
+
     = yield :header_tags
 
     -#  These must come after :header_tags to ensure our initial state has been defined.