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

Conflicts:
- app/controllers/admin/settings_controller.rb
- app/models/form/admin_settings.rb

Conflicts caused by upstream refactoring, while we have
flavours and skins, with the flavour_and_skin pseudo-setting.

app/presenters/instance_presenter.rb

@@ -8,6 +8,7 @@ class InstancePresenter
     :site_description,
     :site_extended_description,
     :site_terms,
+    :closed_registrations_message,
     to: Setting
   )