Allow custom policies to be loaded and exeucuted by the policy filter

Summary: Ref T603. Adds code to actually execute custom policies. (There's still no way to select them in the UI.)

Test Plan:
  - Added and executed unit tests.
  - Edited policies in existing applications.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7292