Implement policies in Phragment
Summary: This implements support for enforcing and setting policies in Phragment.
Test Plan: Set policies and ensured they were enforced successfully.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley
CC: Korvin, epriestley, aran
Maniphest Tasks: T4205
Differential Revision: https://secure.phabricator.com/D7751