New notification type: Subtask changed/closed

A request from my userbase. It would be nice if parent task's assignee could get email notifications when subtasks were closed (e.g. alerting me that I can proceed because the subtask was taken care of)

Right now, I don't see anything like that. I think I'd say distinguish between subtask edited and subtask closed since I could envision wanting to be alerted on one but not the other.