🐛 Fixed missing subscription attribution on free to paid upgrade

[sagzy]

closes https://linear.app/ghost/issue/ENG-1561

• problem: when a free member upgraded to paid, we sometimes did not capture the subscription attribution data
• cause:
  • after checkout, Stripe sends customer.subscription.created, customer.subscription.updated and checkout.session.completed webhook events
  • we want to create a subscription in our database based on the checkout.session.completed event, as it contains additional data (e.g. subscription attribution data)
  • but, we were sometimes creating a subscription based on a customer.subscription.* event during free → paid upgrades, which did not contain subscription attribution data
• solution: we now ignore customer.subscription.* events until a member and its related subscription have been created in the database first, by the checkout.session.completed event