Allow "Custom" policies to be selected in the policy control

Summary: Ref T603. When a user selects "Custom", we pop open the rules dialog and let them create a new rule or edit the existing rule.

Test Plan: Set some objects to have custom policies.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7300