I think we need to run this by @eLobato, as he added it only recently.

As per your e-mail, you can't trivially do `Setting['login_delegation_logout_url'] || controller.extlogout_users_path` as the former has a defined default, but perhaps we can change the default to nil in `app/models/setting/auth.rb` so it only returns a true value if a user's set it (our settings differentiate between defaults and set values).