Make PhabricatorPolicyQuery a CursorPagedPolicyAwareQuery

Summary:
Ref T603. Make these actually implement policy interfaces, so shared infrastructure (like handle loading) works as expected. They don't actually have meaningful policies, and we short circuit all the checks.

(I don't plan to let you set policy controls on policies themselves)

Test Plan: Loaded handles for Policy objects via common infrastructure.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7298