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/concerns/web_app_controller_concern.rb

@@ -7,7 +7,6 @@ module WebAppControllerConcern
     vary_by 'Accept, Accept-Language, Cookie'
 
     before_action :redirect_unauthenticated_to_permalinks!
-    before_action :set_pack
     before_action :set_app_body_class
   end
 
@@ -37,8 +36,4 @@ module WebAppControllerConcern
       end
     end
   end
-
-  def set_pack
-    use_pack 'application'
-  end
 end