message: Disable left/right arrow navigating completions for now
  as the autocomplete is now very eager to appear, this breaks
  caret navigation when typing e.g. anything with a colon.
  Ideally, we should make the AC less eager to appear,
  but this is a quick fix for now.
diff: @@ -682,14 +682,6 @@ export default class MessageComposerInput extends React.Component {
  if (this.autocomplete.countCompletions() > 0) {
  if (!(ev.ctrlKey || ev.shiftKey || ev.altKey || ev.metaKey)) {
  switch (ev.keyCode) {
  - case KeyCode.LEFT:
  - this.autocomplete.moveSelection(-1);
  - ev.preventDefault();
  - return true;
  - case KeyCode.RIGHT:
  - this.autocomplete.moveSelection(+1);
  - ev.preventDefault();
  - return true;
  case KeyCode.UP:
  this.autocomplete.moveSelection(-1);
  ev.preventDefault();
  