Fix non existing field for superuser
`managed_by` field is not set for `superuser` which causes an error
when user tries to edit global permissions.
@@ -79,6 +79,7 @@ class Permission(object):
managed_by = [SUPERUSER] + managed_by
self.inherited_from = inherited_from
+
self.managed_by = managed_by
# Contains the list of arguments required by __call__ to initialise a
