FIX: Calculate no ads for groups server side (#200)

If the selected group to not display ads to had its visibility set to
not be visible then this setting wouldn't work correctly because that
group wouldn't be available client side. The change moves that group
check to be server side so that we can correctly see all the groups that
should not see ads.

plugin.rb

@@ -71,6 +71,10 @@ after_initialize do
     scope.show_adbutler_ads?
   end
 
+  add_to_serializer :current_user, :show_to_groups do
+    scope.show_to_groups?
+  end
+
   class ::AdstxtController < ::ApplicationController
     skip_before_action :preload_json, :check_xhr, :redirect_to_login_if_required