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.
