GREG-17: Duplicate check

This merge request contains a validation check that checks whether two person identities have the same type and value, if the type is not other. This is to avoid duplicates that are not expected in the person identity table.

greg/api/serializers/sponsor.py

@@ -6,4 +6,4 @@ from greg.models import Sponsor
 class SponsorSerializer(serializers.ModelSerializer):
     class Meta:
         model = Sponsor
-        fields = ["id", "feide_id"]
+        fields = ["id", "feide_id", "first_name", "last_name"]