Payment handling

[groenroos]

Built-in integration with Stripe, Braintree or similar developer-friendly payment gateway. Middleware for one-off payments, VAT calculations, PDF invoices, related notifications, and CRUD for credit card details should be provided.

[groenroos]

The current implementation of Stripe is very messy and opinionated. #45 will remove the current implementation, so that we can start completely from scratch on this.

[groenroos]

Moving onto M2, as I want to reprioritise M1 to be strictly features that are necessary for the framework to be functional and useful, and while payment handling is definitely useful, it's not essential.