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