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/sessions_controller.rb

@@ -12,7 +12,6 @@ class Auth::SessionsController < Devise::SessionsController
   skip_before_action :require_functional!
   skip_before_action :update_user_sign_in
 
-  prepend_before_action :set_pack
   prepend_before_action :check_suspicious!, only: [:create]
 
   include Auth::TwoFactorAuthenticationConcern
@@ -104,10 +103,6 @@ class Auth::SessionsController < Devise::SessionsController
 
   private
 
-  def set_pack
-    use_pack 'auth'
-  end
-
   def set_body_classes
     @body_classes = 'lighter'
   end