add client and OS info to amplitude events

closes #7865

website/server/controllers/top-level/payments/stripe.js

@@ -84,6 +84,7 @@ api.checkout = {
         customerId: response.id,
         paymentMethod: 'Stripe',
         sub,
+        headers: req.headers,
       });
     } else {
       let method = 'buyGems';