Improve chat column behavior for newline in empty text box

Summary: Ref T7538. I got this half correct but not fully correct: when you press enter in an empty text box, do nothing (instead of: sending an empty message, or writing a literal newline).

Test Plan: Hit enter in empty chat column box.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7538

Differential Revision: https://secure.phabricator.com/D12089

webroot/rsrc/js/application/conpherence/behavior-durable-column.js

@@ -296,6 +296,10 @@ JX.behavior('durable-column', function(config, statics) {
         return;
       }
 
+      // From here on, interpret this as a "send" action, not a literal
+      // newline.
+      e.kill();
+
       var textarea = _getColumnTextareaNode();
       if (!textarea.value.length) {
         // If there's no text, don't try to submit the form.