closes https://github.com/TryGhost/Product/issues/3595
- when importing paid members with a coupon in Stripe, we currently
search for the corresponding offer in our database and attach it to the
subscription if found. However, if an offer doesn't exist in the
database, we do not create one and don't attach any offer to the
subscription
- with this change, we now support the creation of a new offer, based on
a Stripe coupon, if it didn't exist already
8a32941ae8