Move first-thread selection to the client in Conpherence

Summary: Ref T2421. For mobile, we don't want to select the first thread, since we'll just show a list of threads. Move this behavior to the client and do it when the page is shown on a desktop (or the view is changed to a desktop).

Test Plan: Viewed `/conpherence/`, saw first thread selected. Switched threads.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2421

Differential Revision: https://secure.phabricator.com/D5502