message: Make alt-enter insert new line on macOS
  Alt-enter is a common macOS shortcut for inserting a new line in an
  input that has a separate action tied to the enter key.
diff: @@ -1041,7 +1041,8 @@ export default class MessageComposerInput extends React.Component {
  };
  handleReturn = (ev, change) => {
  - if (ev.shiftKey) {
  + const isMac = navigator.platform.toUpperCase().indexOf('MAC') >= 0;
  + if (ev.shiftKey || (isMac && ev.altKey)) {
  return change.insertText('\n');
  }
  