Convert Conpherence to use normal picture setting flows

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