Implement "USER" policy

Summary: I thought I'd already implemented this, but hadn't. Implement a "USER" policy -- a USER phid means only that user has the capability.

Test Plan: Looked at macros as a user other than the comment owner.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4155