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