Add Room Image to Conpherence header

Summary: This adds the room image to the main header in full Conpherence. It's nice, plus I plan to move the image edit workflow to it to simplify the move to EditEngine. I plan to build some default images for Conpherence which should be better about denoting the room, not just the last person writing.

Test Plan: Click on lots of rooms with and without topics.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16666