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

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

One-sentence description

Add an option to events for users to mark their registration as paid using direct debit.

Motivation

Because we want Thalia Pay (#632).

Desired functionality

Payment option on the user side.

Suggested implementation

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