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!
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