Notify teammates when they are unassigned from a task

Problem:
When a host reassigns a task via the calendar (or task rule change), the ​new​ teammate gets a "Task assigned" notification, but the ​original​ teammate is silently dropped from the task. The task disappears from their iCal feed, but they receive no notification telling them they're no longer responsible. This creates real-world risk of two cleaners arriving at the same property — especially if the original teammate had already seen the initial assignment and put it on their schedule.

Proposed solution:
Add a "Task unassigned" notification type (email + SMS, configurable like other Task notification rules) that fires to the ​previously assigned​ teammate when:

•   A host reassigns the task to someone else, or
•   A task rule change removes them from the task.

Ideally configurable per-property and per-teammate, mirroring the existing Task assigned / Task updated rule structure.

Why it matters:

•   Prevents double-dispatch (two cleaners at one property).
•   Reduces host overhead (no need to manually message the old cleaner).
•   Closes a notification-symmetry gap — we already notify on assignment, we should notify on un-assignment.