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