Improve PolicyFilter and PolicyQuery

Summary:

• Allow PolicyQuery to require specific sets of capabilities other than "CAN_VIEW", like edit, etc. The default set is "view".
• Add some convenience methods to PolicyFilter to test for capabilities.

Test Plan: Viewed pastes, projects, etc. Used other stuff in future diff.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3212