Fix and templatize the logic for external JWT AuthN and registry v2 Auth.

Make it explicit that the registry-v2 stuff is not ready for prime time.

endpoints/v2/manifest.py

@@ -1,3 +1,6 @@
+# XXX This code is not yet ready to be run in production, and should remain disabled until such
+# XXX time as this notice is removed.
+
 import logging
 import re
 import jwt.utils