Because currently Tracker just reffer to particular ack that should be communicated to the user, it is deleted right when notification is deleted.

Steps to reproduce:

1. Install fresh Tracker
2. Create ticket
3. Add an arbitrary ack
4. Remove that ack
5. Run manage.py sendnotifications
6. Check your mailinbox

Nothing will arrive, since there is no ack at all.

That will require formattting before creating notification object, as we do for delete event.