Allow projects to be set as policies

Summary:

• Renames PhabricatorPolicyQuery to PhabricatorPolicyAwareQuery (a query which respects policy settings).
• Introduces PhabricatorPolicyQuery, which loads available policies (e.g., "member of project X").
• Introduces PhabricatorPolicy, which describes a policy.
• Allows projects to be set as policies.
• Allows Paste policies to be edited.
• Covers crazy cases where you make projects depend on themselves or each other because you are a dastardly villan.

Test Plan: Set paste and project policies, including crazy policies like A -> B -> A, A -> A, etc.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3476