gistfile1.txt

/**
 * Implements hook_entity_type_alter().
 * @param \Drupal\Core\Entity\EntityTypeInterface[] $entity_types
 */
function example_contact_form_entity_type_alter(array &$entity_types) {
  $entity_types['contact_message']->setViewBuilderClass('Drupal\example_contact_form\ContactMessageViewBuilder');
}

---

class ContactMessageViewBuilder extends MessageViewBuilder {

}

---

/**
 * Implements hook_mail_alter().
 */
function example_contact_form_mail_alter(&$message) {
  if ($message['module'] == 'contact' && isset($message['params']['contact_message'])) {
    $message['headers']['Content-Type'] = 'text/html';
    $render = entity_view($message['params']['contact_message'], 'mail');
     $message['body'] = array(
      render($render),
    );
  }
}