Don't mark a thread as seen if durable column is minimized

Summary: More work to do here on the JS side, but this at least makes sure users with a small chat window have some notification marked that new replies have not been seen.

Test Plan: Open two windows. Window 1 has durable minimized, Window 2 is full conpherence. Send a message from Window 2, see header count in Window 1 increase. Repeat with durable open, see no change in window.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16650