R1:fbfccf5ddc08

Improve Policy options

Summary:
- Add an "Administrators" policy.
- Allow "Public" to be completely disabled in configuration.
- Simplify unit tests, and cover the new policies.

Test Plan: Ran unit tests.

Reviewers: btrahan, vrana, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D2238