Wipe textarea immediately when submitting a new message in Conpherence

Summary: Fixes T11791. We do this in durable column, but not in regular Conpherence. I think this is the right place? Not sure how this will feel with high lag.

Test Plan: Submit lots of text in a Conpherence.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11791

Differential Revision: https://secure.phabricator.com/D16969