Conpherence - add support for users with no conpherences in durable column view

Summary: Ref T7014. This just makes it so there's almost no UI and a simple "You have no messages. <button>Send a message.</button>" UI

Test Plan: hacked the code such that should_404 and conpherence were false and null respectively. verified i got the right ui in the durable column. verified send a message button worked, ending up with me in main conpherence view on the right message

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7014

Differential Revision: https://secure.phabricator.com/D12042