add url to the notifications API

[PetrDlouhy]

This MR adds URL of the target, action and actor url to the API endpoint (if the url is defined by get_url_for_notifications(notification, request) or get_absolute_url() method on the object).

We would like to make notifications loaded through the JS code with links to the target objects.

We wanted to add plain get_absolute_url() to the API endpoint, but we realized, that it is not sufficient in case where the link should depend also on the user that is accessing the notification, so we needed to add the request parameter (and we also added the notification object).