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/challenges_controller.rb

@@ -5,7 +5,6 @@ class Auth::ChallengesController < ApplicationController
 
   layout 'auth'
 
-  before_action :set_pack
   before_action :authenticate_user!
 
   skip_before_action :check_self_destruct!
@@ -21,10 +20,4 @@ class Auth::ChallengesController < ApplicationController
       render_challenge
     end
   end
-
-  private
-
-  def set_pack
-    use_pack 'auth'
-  end
 end