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

Ghost User
2019-11-06T19:31:19+01:00

Subtask of #632, partially blocked by #930.

### 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!