Changelog: Billing

Existing customers can now choose saved payment methods when using Stripe Checkout for subscriptions.

Stripe Checkout no longer errors in setup mode when creating sessions; saved payment method options are omitted for setup and included for subscription mode.

Slack subscription notifications now show the billing interval (e.g. 1-month, 10-months) so amounts are not misleading for pre-sold subscriptions.

Added notifications for subscription created, phase changes, and pre-sold payment method needed

Added native Stripe connected account settings page, replacing the legacy iframe integration