R1:1c222c88349e

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".