Work around a bug in PHP 5.3-ish with abstract methods in interfaces

Summary:
@chad is hitting an issue described in P961, which I think is this bug in PHP: https://bugs.php.net/bug.php?id=43200

Work around it by defining a "PHIDInterface" and having both "Flaggable" and "Policy" extend it, so that there is only one `getPHID()` declaration.

Test Plan: shrug~

Reviewers: chad, btrahan

Reviewed By: chad

CC: chad, aran

Differential Revision: https://secure.phabricator.com/D7408