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
refs https://github.com/TryGhost/Team/issues/1726
- updates offer setup to allow new `trial` as discount type, was prev only `fixed` and `percent`
- updates offer setup to allow `amount` as free trial days value
- updates offer setup to allow `trial` as discount duration value for trial offers, was prev only `once`/`forever`/`repeating`
refs https://github.com/TryGhost/Team/issues/1236
We want to be able to use the OfferName as the name property for a
Stripe Coupon - which has a maximum character length of 40.
