Replace use_pack, layouts/theme and @theme by new helpers

Packs are now loaded from views, just like upstream, and are
identified by their filenames. The definition of `theme.yml` has
changed as such:
- `pack_directory` is now required
- `pack` is now unused
- `signed_in_preload` has been introduced

app/controllers/statuses_cleanup_controller.rb

@@ -6,7 +6,6 @@ class StatusesCleanupController < ApplicationController
   before_action :authenticate_user!
   before_action :set_policy
   before_action :set_body_classes
-  before_action :set_pack
   before_action :set_cache_headers
 
   def show; end
@@ -27,10 +26,6 @@ class StatusesCleanupController < ApplicationController
 
   private
 
-  def set_pack
-    use_pack 'settings'
-  end
-
   def set_policy
     @policy = current_account.statuses_cleanup_policy || current_account.build_statuses_cleanup_policy(enabled: false)
   end