Fix a race condition with the chat column

Summary:
Ref T7062. When we load a thread, we always show the column, even if it has been closed between the time we sent the request and when we're processing the response.

Normally this isn't a big deal, but it can specifically show up on mobile.

(This load also shouldn't be happening at all, but I'll fix that separately.)

Test Plan: Mobile no longer shows the column after it loads.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7062

Differential Revision: https://secure.phabricator.com/D12195