Send on enter in Conpherence chat column

Summary: Ref T7538. We can figure out whether to backport this to main Conpherence later and/or remove buttons, etc., but this behavior seems pretty clearly good.

Test Plan:

• Pressed enter (sent message).
• Pressed shift+enter (newline).

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7538

Differential Revision: https://secure.phabricator.com/D12085