Plan groups and pricing versions
Plans became versioned, so changing pricing no longer rewrites what existing subscribers are on.
Added
3- BillingPlans are organized into groups with versioned pricing, so publishing new prices leaves existing subscriptions on the version they signed up to.
- BillingSubscription items and categories, and Stripe credentials and webhooks configurable per organization.
- AudienceDate-typed attributes with matching filters, and a record of how an attribute changed over time.