refs https://github.com/TryGhost/Team/issues/586
We have to use `belongsToMany` because of the way bookshelf relations
work. In reality the relationship is 'hasMany', e.g. a Product has many
Stripe Prices.
These relations are the minimal needed to satisfy the following
relationships without transforming the results. (e.g. flattening the
StripePrices from a list of StripeProducts for a Product)
Product -> StripeProduct: product.related('stripeProducts')
StripeProduct -> StripePrice: stripeProduct.related('stripePrices');
Product -> StripePrice: product.related('stripePrices');
StripePrice -> Product: stripePrice.related('stripeProduct.product');
