Allow editPolicy, viewPolicy, and repositoryPHID to be edited from the web UI in Differential

Summary: Ref T603. I think T2222 is fraught with peril so I'm not going to try to sequence it ahead of T603 for Differential. Provide access to policy controls in Differential's edit view.

Test Plan: See screenshot.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7142