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/passwords_controller.rb

@@ -3,7 +3,6 @@
 class Auth::PasswordsController < Devise::PasswordsController
   skip_before_action :check_self_destruct!
   before_action :redirect_invalid_reset_token, only: :edit, unless: :reset_password_token_is_valid?
-  before_action :set_pack
   before_action :set_body_classes
 
   layout 'auth'
@@ -32,8 +31,4 @@ class Auth::PasswordsController < Devise::PasswordsController
   def reset_password_token_is_valid?
     resource_class.with_reset_password_token(params[:reset_password_token]).present?
   end
-
-  def set_pack
-    use_pack 'auth'
-  end
 end