Conpherence - turn on mentions interface for Conpherence rooms / messages

Summary: Fixes T7756. This is the last little stray bit, though finishing T7757 also helps this feature IMO.

Test Plan: said "ZXX is the best" in comment on DXX and saw proper mention transaction on ZXX

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7756

Differential Revision: https://secure.phabricator.com/D12405