Make the "you can't edit away your edit capability" policy check generic

Summary:
Ref T4379. Currently, you can edit away your edit capability in Projects. Prevent this in a general way.

Since some objects have complex edit policies (like "the owner can always edit"), we can't just check the value itself. We also can't fairly assume that every object has a setEditPolicy() method, even though almost all do right now. Instead, provide a way to pretend we've completed the edit and changed the policy.

Test Plan: Unit tests, tried to edit away my edit capability.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4379

Differential Revision: https://secure.phabricator.com/D8179