Prevent extraneous subscriptions

[9larsons]

ref https://linear.app/tryghost/issue/ONC-38/

• Stripe checkout webhooks should only create members for products that exist in Ghost
• Ghost makes assumptions about the default product that do not stand up to Stripe having extraneous subscription products

We found that members were being created when customers used Stripe for subscriptions that are for products unrelated to Ghost (shared Stripe account). This problem is two-fold: 1) these members have not been to/know of the Ghost site and 2) the subscription caused the member to be linked to the default subscription while using the price of the paid for subscription, causing errors in MRR reporting.