getFilename() {
    var userProvidedFilename = $('.filename').val();
    if (userProvidedFilename) {
      return userProvidedFilename;
    } else {
      return 'Lights_Export_' +
          (new Date()).toISOString().replace(/[^0-9]/g, '');
    }
  }
Returns the filename entered in the input box or a generated default
filename if none.
@return {string} The name to give the exported file in the user's Drive.