Make the policy control a JS dropdown with icons

Summary: Ref T603. After thinking about this for a bit I can't really come up with anything better than what Facebook does, so I'm going to implement something similar for choosing custom policies. To start with, swap this over to a JS-driven dropdown.

Test Plan: See screenshot.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7285