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

Conflicts:
- `app/models/account.rb`:
  Not a real conflict, just upstream getting rid of unused constants too close
  to glitch-soc-specific contents.
  Removed unused constants like upstream did.
- `app/models/trends.rb`:
  Conflict because glitch-soc disabled email notifications for trending links.
  Upstream has refactored this quite a bit and added trending posts.
  Took upstream code, but disabling the extra trending stuff will come in
  another commit.
- `app/views/admin/trends/links/index.html.haml`:
  Conflict due to glitch-soc's theming system.
  Ported upstream changes accordingly.

app/controllers/concerns/localized.rb

@@ -27,4 +27,8 @@ module Localized
   def available_locale_or_nil(locale_name)
     locale_name.to_sym if locale_name.present? && I18n.available_locales.include?(locale_name.to_sym)
   end
+
+  def content_locale
+    @content_locale ||= I18n.locale.to_s.split(/[_-]/).first
+  end
 end