GREG-21: Add foedselsnummer as type

Adding fødselsnummer as an identification type.

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"]