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/settings/two_factor_authentication/webauthn_credentials_controller.rb

@@ -85,10 +85,6 @@ module Settings
 
       private
 
-      def set_pack
-        use_pack 'auth'
-      end
-
       def redirect_invalid_otp
         flash[:error] = t('webauthn_credentials.otp_required')
         redirect_to settings_two_factor_authentication_methods_path