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/base_controller.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 class Settings::BaseController < ApplicationController
-  before_action :set_pack
   layout 'admin'
 
   before_action :authenticate_user!
@@ -10,10 +9,6 @@ class Settings::BaseController < ApplicationController
 
   private
 
-  def set_pack
-    use_pack 'settings'
-  end
-
   def set_body_classes
     @body_classes = 'admin'
   end