feat: add admin user serializer with accept invitation url

packages/backend/src/serializers/admin/user.js

@@ -0,0 +1,11 @@
+import userSerializer from '../user.js';
+
+const adminUserSerializer = (user) => {
+  const userData = userSerializer(user);
+
+  userData.acceptInvitationUrl = user.acceptInvitationUrl;
+
+  return userData;
+};
+
+export default adminUserSerializer;