billing: Use UserProfile.id to ascertain customer's email.

Previously, our Stripe webhook event handler code retrieved the
user's email from Stripe using the stripe.Customer.email attribute.
This led to situations such that whenever the email that Stripe had
did not correspond to a UserProfile in Zulip, the payment flow
failed since we couldn't find a UserProfile associated with the
given email.

Now, we pass in the UserProfile.id in the metadata to Stripe's
checkout Session object, so that we can fetch the correct email
in future Stripe requests.

corporate/lib/stripe_event_handler.py

@@ -12,7 +12,7 @@ from corporate.lib.stripe import (
     update_or_create_stripe_customer,
 )
 from corporate.models import Event, PaymentIntent, Session
-from zerver.models import get_user_by_delivery_email
+from zerver.models import get_active_user_profile_by_id_in_realm, get_user_by_delivery_email
 
 billing_logger = logging.getLogger("corporate.stripe")
 
@@ -70,9 +70,10 @@ def handle_checkout_session_completed_event(
     session.save()
 
     stripe_setup_intent = stripe.SetupIntent.retrieve(stripe_session.setup_intent)
-    stripe_customer = stripe.Customer.retrieve(stripe_setup_intent.customer)
     assert session.customer.realm is not None
-    user = get_user_by_delivery_email(stripe_customer.email, session.customer.realm)
+    user_id = stripe_session.metadata.get("user_id")
+    assert user_id is not None
+    user = get_active_user_profile_by_id_in_realm(user_id, session.customer.realm)
     payment_method = stripe_setup_intent.payment_method
 
     if session.type in [