Allow application policies to be edited

Summary:
Ref T603. Enables:

  - Application policies can be edited.
  - Applications can define custom policies (this will be used for setting defaults, like "what is the default visibiltiy of new tasks", and meta-policies, like "who can create a task?").

Test Plan: Edited application policies. A future diff does more with custom policies.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7205