Raise an event after a user has been deleted

Currently we have the on_user_delete event: it can cancel the deletion of
a user.
This is fine, but we don't have an event that's being raised when a user
is effectively deleted.

Let's add it...
Raise an event after a user has been deleted v2

Currently we have the on_user_delete event: it can cancel the deletion of
a user.
This is fine, but we don't have an event that's being raised when a user
is effectively deleted.

Let's add it...
Merge pull request #1769 from mlocati/add-user-deleted-event

Raise an event after a user has been deleted