Make Conpherence threads update in real time, very roughly

Summary:
Ref T4083. This needs some work (mostly in the Conpherence JS itself), but is sort of functional. In particular:

  - On thread pages, add the thread as a `pageObject`.
  - After updating a thread, send a new "message" event to the server.
  - Share a little more event posting code.
  - In the browser, use event dispatch to respond to events.
  - Add a listener for the new event type.
  - Update conpherence threads (this part is really yucky).

Test Plan: With multiple browser windows / browsers open, posted a message to a thread, and saw it update everywhere.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: chad, epriestley

Maniphest Tasks: T4083

Differential Revision: https://secure.phabricator.com/D9486