[OAuth] Fix groups claim

Closes #551.

aleksis/core/util/auth_helpers.py

@@ -82,7 +82,7 @@ class CustomOAuth2Validator(OAuth2Validator):
             }
 
         if "groups" in request.scopes and has_person(request.user):
-            claims["groups"] = request.user.person.groups.values_list("name", flat=True).all()
+            claims["groups"] = request.user.person.member_of.values_list("name", flat=True).all()
 
         return claims