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/auth/setup_controller.rb

@@ -3,7 +3,6 @@
 class Auth::SetupController < ApplicationController
   layout 'auth'
 
-  before_action :set_pack
   before_action :authenticate_user!
   before_action :require_unconfirmed_or_pending!
   before_action :set_body_classes
@@ -43,8 +42,4 @@ class Auth::SetupController < ApplicationController
   def user_params
     params.require(:user).permit(:email)
   end
-
-  def set_pack
-    use_pack 'sign_up'
-  end
 end