function mapResponse_(response) {
  let initialValue = {
    email: response.getRespondentEmail(),
    timestamp: response.getTimestamp(),
  };
  return response.getItemResponses().reduce(function(obj, itemResponse) {
    let key = itemResponse.getItem().getTitle();
    obj[key] = itemResponse.getResponse();
    return obj;
  }, initialValue);
}
Converts a form response to an object keyed by the item titles. Allows easier
access to response values.

@param {FormResponse} response
@return {Object} Form values keyed by question title