refs https://github.com/TryGhost/Team/issues/1166
This is a new module which will eventually handle all payment related
things. This allows the Offers module to focus exclusively on the Ghost
concepts, and the Payments module will handle the association between
Offer & Stripe Coupon, Tier & Stripe Product, Cadence & Stripe Price.
This decoupling allows us to not have to consider the lack of Stripe
data for an Offer, which is the case after a Stripe Disconnect. Instead
all of the population/repopulation/lazy-creating can be handled here.
