refs https://github.com/TryGhost/Team/issues/1519
- Added offer repository dependency to member repository (offerAPI didn't work because it creates a new transaction that resulted in a deadlock during tests)
- Store the offer id from the Stripe subscription metadata in the subscription (only if the discount is still active)
- Also added the offer id to the metadata for a Stripe coupon, this will make adding and removing coupons a bit more foolproof
- Prefer the usage of the offer metadata from a coupon if it is present
- When no discount is applied to a subscription, it always sets the offer id to null, even when the metadata still contains the offer
- The offer_id remains stored when a subscription is canceled/expired
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