refs https://github.com/TryGhost/Team/issues/1132
We have to include the Offer on the metadata for the Stripe Checkout -
as Offers with a duration of 'once' will not always be present on the
Subscription after fetching it.
Once we receive the Stripe Checkout webhook we emit an event for
subscription created - the reason we use an event is because this logic
should eventually live in a Payments/Stripe module - and we'd want to
decouple it from the Members module.
The Members module is in charge of writing Offer Redemptions - rather
than the Offers module - because Offer Redemptions are "owned" by a
Member - and merely reference and Offer. Eventually Offer Redemptions
could be replaced by Subscriptions.
