R1:77221bee72cb

Allow objects to specify custom policy unlocking behavior, and tasks to have owners unlocked

Summary: Depends on D20256. Ref T13249. See PHI1115. This primarily makes `bin/policy unlock --owner epriestley T123` work. This is important for "Edit Locked" tasks, since changing the edit policy doesn't really do anything.

Test Plan: Hard-locked a task as "alice", reassigned it to myself with `bin/policy unlock --owner epriestley`.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13249…