GREG-21: Various model updates

The commits in this merge request contain changes done to be able to import the test data. It is basically opening up for more fields to be able to be null.

Maybe @jbr also wants to take a quick look to see if he agrees with the fields that now can be null.

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"]