Create options for redirecting to Mercury Editor after node save

This functionality is ready for review. (see MR)

I've added a comprehensive test for the new config options for redirecting.

The most questionable code:

  $request = \Drupal::request();
  if ($request->query->has('destination')) {
    $request->query->remove('destination');
  }

This is required in the case the user clicks edit from /admin/content and the View is configured to append the destination parameter. Not sure if we want an additional config option like "Ignore Destination"...maybe overkill?