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/admin/base_controller.rb

@@ -7,7 +7,6 @@ module Admin
 
     layout 'admin'
 
-    before_action :set_pack
     before_action :set_body_classes
     before_action :set_cache_headers
 
@@ -19,10 +18,6 @@ module Admin
       @body_classes = 'admin'
     end
 
-    def set_pack
-      use_pack 'admin'
-    end
-
     def set_cache_headers
       response.cache_control.replace(private: true, no_store: true)
     end