add sponsorship id to sponsor list api endpoint

this will be useful to make the sync more reliable to link via id not string

Author: ewdurbin

sponsors/api.py

@@ -34,6 +34,7 @@ def get(self, request, *args, **kwargs):
         for sponsorship in sponsorships.select_related("sponsor").iterator():
             sponsor = sponsorship.sponsor
             base_data = {
+                "sponsor_id": sponsor.id,
                 "sponsor": sponsor.name,
                 "sponsor_slug": sponsor.slug,
                 "level_name": sponsorship.level_name,