def add_next(self, url, context):
        """
        With both `redirect_field_name` and `redirect_field_value` available in
        the context, add on a querystring to handle "next" redirecting.
        """
        if all([key in context for key in ["redirect_field_name", "redirect_field_value"]]):
            if context["redirect_field_value"]:
                url += "?" + urlencode({
                    context["redirect_field_name"]: context["redirect_field_value"],
                })
        return url
With both `redirect_field_name` and `redirect_field_value` available in
        the context, add on a querystring to handle "next" redirecting.