stripe: Link InvoiceItem to Invoice.

This is required for upgrading to the basil version of stripe.
Changes haven't been tested in this commit. It is just for easy
verification of changes.

corporate/tests/test_stripe.py

@@ -4736,13 +4736,6 @@ class StripeTest(StripeTestCase):
         invoices = []
         assert customer.stripe_customer_id is not None
         for _ in range(num_invoices):
-            stripe.InvoiceItem.create(
-                amount=10000,
-                currency="usd",
-                customer=customer.stripe_customer_id,
-                description="Zulip Cloud Standard",
-                discountable=False,
-            )
             invoice = stripe.Invoice.create(
                 auto_advance=True,
                 collection_method="send_invoice",
@@ -4750,6 +4743,16 @@ class StripeTest(StripeTestCase):
                 days_until_due=DEFAULT_INVOICE_DAYS_UNTIL_DUE,
                 statement_descriptor="Zulip Cloud Standard",
             )
+            assert invoice.id is not None
+            stripe.InvoiceItem.create(
+                invoice=invoice.id,
+                amount=10000,
+                currency="usd",
+                customer=customer.stripe_customer_id,
+                description="Zulip Cloud Standard",
+                discountable=False,
+            )
+
             stripe.Invoice.finalize_invoice(invoice)
             invoices.append(invoice)
         return invoices