ref https://linear.app/tryghost/issue/ONC-199
The `updateSubscriptionItemPrice()` method in our Stripe library used by the importer when moving a subscription over to a Ghost product/price was setting `proration_behavior: 'always_invoice'`. This resulted in invoices being created when changing the subscription (even though no prices were changing as far as the customer is concerned) and in some cases where a customer previously had a one-off discount the customer was incorrectly charged the proration difference because the discount was no longer applied to the new invoice.
- updated `updateSubscriptionItemPrice()` to accept an `options` param allowing the `proration_behavior` property passed to the Stripe API to be overridden on a per-call basis
- updated the `forceStripeSubscriptionToProduct()` method used by the importer to pass an options object with `prorationBehavior: 'none'` when updating the subscription item price so that no invoice and no unexpected charges occur when importing
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