Prepare the policy rule edit endpoint for integration
Summary: Ref T603. Allow the endpoint to take an existing policy PHID to populate the editor and return a useful datastructure.
Test Plan: In the next revision, actually hooked this up.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T603
Differential Revision: https://secure.phabricator.com/D7299