Returns a string containing summary info for all GRR users.

def GetAllUserSummaries():
  """Returns a string containing summary info for all GRR users."""
  grr_api = maintenance_utils.InitGRRRootAPI()
  user_wrappers = sorted(grr_api.ListGrrUsers(), key=lambda x: x.username)
  summaries = [_Summarize(w.data) for w in user_wrappers]
  return "\n\n".join(summaries)