Conpherence - add isRoom column to thread table

Summary: Fixes T7583. We also add key_room, which uses isRoom and dateModified since a very common view of rooms is going to be ordered by last updated.

Test Plan: made the conpherence view controller query specify withIsRoom(true) and withIsRoom(false). The former made the controller correctly 404 while the latter had no change in functionality.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7583

Differential Revision: https://secure.phabricator.com/D12102