Import the plugins once only.

def RunOnce(self):
    """Import the plugins once only."""
    # pylint: disable=unused-variable,g-import-not-at-top
    from grr_response_server.gui import gui_plugins
    # pylint: enable=unused-variable,g-import-not-at-top

    if config.CONFIG.Get("AdminUI.django_secret_key", None):
      logging.warning(
          "The AdminUI.django_secret_key option has been deprecated, "
          "please use AdminUI.csrf_secret_key instead.")