Have all tag code add, modify and delete both old and new style tags

This preps us for being able to use the new data model with existing repositories

data/registry_model/registry_pre_oci_model.py

@@ -522,6 +522,7 @@ class PreOCIModel(SharedModel, RegistryDataInterface):
     try:
       tag_manifest, _ = model.tag.associate_generated_tag_manifest(namespace_name, repo_name,
                                                                    tag.name, manifest, storage_map)
+      assert tag_manifest
     except IntegrityError:
       tag_manifest = model.tag.get_tag_manifest(tag_obj)