Thalia Pay: Add an option to pizzas for users to mark their order as paid using direct debit.

Subtask of #632, partially blocked by #930 (closed).

One-sentence description

Add an option to pizzas for users to mark their order as paid using direct debit.

Motivation

Because we want Thalia Pay (#632).

Desired functionality

Payment option on the user side. And option to see the value in the admin.

Suggested implementation

Have a button on the pizzas page that has the user confirm their payment or redirect to information about direct debits. Also provide an API for the app!