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!
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information