AJAX callback to refresh form.
array $form:
FormStateInterface $form_state:
array Form element to replace.
public function ajaxCallback(array &$form, FormStateInterface $form_state) {
$source = $form_state
->getValue('source');
$response = new AjaxResponse();
$response
->addCommand(new RedirectCommand($this
->getUrlForSource($source)
->setAbsolute()
->toString()));
return $response;
}