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/oauth/authorized_applications_controller.rb

@@ -5,7 +5,6 @@ class Oauth::AuthorizedApplicationsController < Doorkeeper::AuthorizedApplicatio
 
   before_action :store_current_location
   before_action :authenticate_resource_owner!
-  before_action :set_pack
   before_action :require_not_suspended!, only: :destroy
   before_action :set_body_classes
   before_action :set_cache_headers
@@ -31,10 +30,6 @@ class Oauth::AuthorizedApplicationsController < Doorkeeper::AuthorizedApplicatio
     store_location_for(:user, request.url)
   end
 
-  def set_pack
-    use_pack 'settings'
-  end
-
   def require_not_suspended!
     forbidden if current_account.unavailable?
   end