Add natural keys to the MetadataType and MetadataSet models, make the metadata set label unique

apps/metadata/managers.py

@@ -0,0 +1,13 @@
+from __future__ import absolute_import
+
+from django.db import models
+
+
+class MetadataTypeManager(models.Manager):
+    def get_by_natural_key(self, name):
+        return self.get(name=name)
+
+
+class MetadataSetManager(models.Manager):
+    def get_by_natural_key(self, name):
+        return self.get(name=name)