backCallbackFunction = function () {
      firebaseui.auth.widget.handler.common.handleSignInStart(
          app, container);
  }
Handles unauthorized users.

@param {!firebaseui.auth.AuthUI} app The current Firebase UI instance whose
    configuration is used.
@param {!Element} container The container DOM element.
@param {?string} userIdentifier The user identifier of the account, can be
    email address or phone number. When not determinable (federated sign-in
    flows), this is null.
@param {?string} provider The provider used for sign-in, if determinable
    (null otherwise).