Return the class pointed to be an app setting variable.

def import_settings_class(setting_name):
    """
    Return the class pointed to be an app setting variable.
    """
    config_value = getattr(settings, setting_name)
    if config_value is None:
        raise ImproperlyConfigured("Required setting not found: {0}".format(setting_name))

    return import_class(config_value, setting_name)