In Conpherence, stop throwing away stuff users have typed when a reply arrives

Summary:
Ref T12562. I think the pre-send-on-enter behavior was: disable textarea, send message, clear area on response?

That got changed but not completely, maybe. There's currently an issue here:

• Add a sleep(3) to UpdateController.
• Type "AAA".
• Press enter.
• Real fast, type "BBB".
• When the "AAA" arrives, your "BBB" is lost. Sad!

Test Plan:

• Did the thing described above; no longer lost "BBB".
• Switched threads, sent messages, couldn't find anything else this breaks. It dates from a long time ago so I think it's just pre-SOE stuff.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12562

Differential Revision: https://secure.phabricator.com/D17742