New NUX states for Conpherence

Summary: Roughly, if user isn't in any rooms, search for joinable ones. If no results, show big NUX banner.

Test Plan: Left all rooms, got fallback, joined room, left room. Create new instance, see new NUX. Set instance to public, visit Conpherence with and without public rooms.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16563