Ensure that verbs cannot be performed on disabled namespaces or by disabled users

endpoints/verbs/models_interface.py

@@ -152,3 +152,8 @@ class VerbsDataInterface(object):
     or None if none.
     """
     pass
+
+  @abstractmethod
+  def is_namespace_enabled(self, namespace_name):
+    """ Returns whether the given namespace exists and is enabled. """
+    pass