Conpherence - people widget

Summary:
adds ye olde people widget. Features include

 - handle-based display, so we get status for free. (Note less pretty than M14 would have it!)
 - can add a person
 - can remove a person
 - can see the people already in the conpherence

Test Plan: added and removed people and noted they joined / re-added as appropriate. Tried to add someone already in the conpherence and got a "transaction has no effect" message

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2399

Differential Revision: https://secure.phabricator.com/D5466