GREG-91: Adding units as part of sponsor listing

greg/api/serializers/sponsor.py

 from rest_framework import serializers
 
+from greg.api.serializers.organizational_unit import OrganizationalUnitSerializer
 from greg.models import Sponsor
 
 
 class SponsorSerializer(serializers.ModelSerializer):
+    units = OrganizationalUnitSerializer(many=True, read_only=True)
+
     class Meta:
         model = Sponsor
-        fields = ["id", "feide_id", "first_name", "last_name"]
+        fields = ["id", "feide_id", "first_name", "last_name", "units"]