message: Fix redo keyboard shortcut on macOS
  I added the correct macOS shortcut for redo which is Cmd+Shift+Z. I had to reorder the if statement so redo is checked first, otherwise it would detect the undo first and never check that I was actually trying to redo.
diff: @@ -370,6 +370,17 @@ export default class BasicMessageEditor extends React.Component {
  } else if (modKey && event.key === Key.GREATER_THAN) {
  this._onFormatAction("quote");
  handled = true;
  + // redo
  + } else if ((modKey && event.key === Key.Y) ||
  + (IS_MAC && event.shiftKey && event.key === Key.Z)) {
  + debugger;
  + if (this.historyManager.canRedo()) {
  + const {parts, caret} = this.historyManager.redo();
  + // pass matching inputType so historyManager doesn't push echo
  + // when invoked from rerender callback.
  + model.reset(parts, caret, "historyRedo");
  + }
  + handled = true;
  // undo
  } else if (modKey && event.key === Key.Z) {
  if (this.historyManager.canUndo()) {
  @@ -379,15 +390,6 @@ export default class BasicMessageEditor extends React.Component {
  model.reset(parts, caret, "historyUndo");
  }
  handled = true;
  - // redo
  - } else if (modKey && event.key === Key.Y) {
  - if (this.historyManager.canRedo()) {
  - const {parts, caret} = this.historyManager.redo();
  - // pass matching inputType so historyManager doesn't push echo
  - // when invoked from rerender callback.
  - model.reset(parts, caret, "historyRedo");
  - }
  - handled = true;
  // insert newline on Shift+Enter
  } else if (event.key === Key.ENTER && (event.shiftKey || (IS_MAC && event.altKey))) {
  this._insertText("\n");
  