R1:7d9687057056

Maniphest - use subscribers framework properly

Summary: Fixes T5604. This should fix some random bugs, lets us move forward more easily, and all that good stuff about killing code debt.

Test Plan:
- Conduit method maniphest.createtask
- verified creating user subscribed
- verified subscription transaction
- Conduit method maniphest.update
- verified subscribers set as specified to ccPHIDs parameter
- verified subscription transaction
- Herald
- verified herald rule to add subscriber worked…