fixes https://github.com/TryGhost/Product/issues/3728
- When importing members from Stripe with an existing offer, that didn't
exist in Ghost, the offer never got linked with the imported
subscription because of a missing return statement.
- Fixes importing offers with duplicate names
- Added E2E tests for creating members from a Stripe Customer ID
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
