Conpherence - grey out username mentions if they aren't in the conpherence

Summary: Fixes T7578. This was pretty easy because conpherence funnels all transacton stuff through this spot

Test Plan: made a new room so only my user was a participant. wrote "@myself will work and @anotherguy will be greyed out" and so it was as expected

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7578

Differential Revision: https://secure.phabricator.com/D12114