Previously, when a user canceled a subscription, the access would revoke
immediately on Discourse vs. at the end of the billing period. This
commit changes the behavior to remove membership at the end of the
billing period using Stripe's `cancel_at_period_end` attribute on the
Subscription object.
This commit now requires the setup of webhooks for subscription
processing to occur correctly.
Building off the foundation of using the Prices API, this PR adds the ability to create a one-time purchase plan for any product, which then can add a user to the specified plan group.
Some things to be aware of:
One-time purchases cannot have trials.
One-time purchases use the Invoice API instead of Subscriptions. Invoices are created then charged immediately.
Users should receive emails for these invoices directly from Stripe just like subscriptions.
