R1:3de3a72dd8c7

Add a "Subscribers" object policy

Summary:
Ref T5681. Getting this to work correctly is a bit tricky, mostly because of the policy checks we do prior to applying an edit.

I think I came up with a mostly-reasonable approach, although it's a little bit gross. It uses `spl_object_hash()` so it shouldn't be able to do anything bad/dangerous (the hints are strictly bound to the hinted object, which is a clone that we destroy moments later).

Test Plan:
- Added + ran a unit test.
- Created a task with a…