🐛 Fixed missing subscription attribution on free to paid upgrade (#21846

)

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