Summary: This moves room pictures out of the dialog and into it's own PictureController. Also adds a standard image (and removes the "last person to chat" picture (though we could add that back. My plan is though that direct messages use auto use the other person's photo, after we have editengine and room pictures will have a plain, replaceable image.
Test Plan: Set a new room picture, remove a picture. Run migration, see old images properly set with new image.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Maniphest Tasks: T11730
Differential Revision: https://secure.phabricator.com/D16669
