By doing so and including it in ActionDispatch::Routing::UrlFor, we make
it available in controllers, helpers and specs, and so we can remove the
duplication we had there with methods dealing with the same problem.
Even if monkey-patching is ugly, using a different module and executing
ActionDispatch::Routing::UrlFor.send(:include, MyModule) wouldn't make
the method available in the controller.
Notifications usually link to the associated notifiable, but the new
AdminNotifications have a link attribute that may be empty or contain an
external or internal url.
