Allow People to be Flagged

Summary: Ref T8888, Makes People Flaggable (and makes me wonder if we should rename Flags->Bookmarks).

Test Plan: Flag myself. Get excited.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: joshuaspence, epriestley, Korvin

Maniphest Tasks: T8888

Differential Revision: https://secure.phabricator.com/D13654