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

Differential Revision: https://secure.phabricator.com/D20257