Prevent registry operations against disabled namespaces

Allows admins to completely wall off a namespace by disabling it

Fixes https://jira.coreos.com/browse/QUAY-869

endpoints/v2/models_pre_oci.py

@@ -258,6 +258,10 @@ class PreOCIModel(DockerRegistryV2DataInterface):
     except model.InvalidManifestException:
       return
 
+  def is_namespace_enabled(self, namespace_name):
+    namespace = model.user.get_namespace_user(namespace_name)
+    return namespace is None or namespace.enabled
+
 
 def _docker_v1_metadata(namespace_name, repo_name, repo_image):
   """