Allow task policies to be edited from the UI; show policy information on the detail page

Summary:
Ref T603. Adds policy controls to the task edit UI.

@chad, status + policy renders a little weird -- did I mess something up? See screenshot.

Test Plan: Edited policies, viewed a task.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7123