Infers user name and subject urn from self.urn.

def InferUserAndSubjectFromUrn(self):
    """Infers user name and subject urn from self.urn."""
    _, hunts_str, hunt_id, user, _ = self.urn.Split(5)

    if hunts_str != "hunts":
      raise access_control.UnauthorizedAccess(
          "Approval object has invalid urn %s." % self.urn,
          requested_access=self.token.requested_access)

    return (user, aff4.ROOT_URN.Add("hunts").Add(hunt_id))