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/relationships_controller.rb

@@ -5,7 +5,6 @@ class RelationshipsController < ApplicationController
 
   before_action :authenticate_user!
   before_action :set_accounts, only: :show
-  before_action :set_pack
   before_action :set_relationships, only: :show
   before_action :set_body_classes
   before_action :set_cache_headers
@@ -73,10 +72,6 @@ class RelationshipsController < ApplicationController
     @body_classes = 'admin'
   end
 
-  def set_pack
-    use_pack 'admin'
-  end
-
   def set_cache_headers
     response.cache_control.replace(private: true, no_store: true)
   end