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_controller.rb

@@ -41,7 +41,6 @@ class StatusesController < ApplicationController
   end
 
   def embed
-    use_pack 'public'
     return not_found if @status.hidden? || @status.reblog?
 
     expires_in 180, public: true