Describe the bug

Since #3417 it's not possible to delete a Member when they've made a payment.

For privacy reasons, the user coudl request being deleted. While we do need to maintain payment history (so the old CASCADE of payments was not correct either), I think we can probably be fine allowing the delete with SET_NULL on Payment.paid_by.

I'm reasonably confident this will work fine:

• BankAccounts (including sepa mandates) contain a copy of the real name, and have SET_NULL on they FK to the user. So we still keep any sepa mandate that we're legally required to store.
• Payments don't need to have a payer (we do currently payments without a known payer, and can push those to moneybird).

Something we also need to fix is this CASCADE, and the equivalent in FoodOrder:

Basically, if we're deleting a user, we shouldn't CASCADE objects that in the normal dataminimisation simply get their FK to Member set to null. This goes for eventregistrations but also FoodOrder.

How to reproduce

1. Try to delete a user.
2. It's not allowed because of protected foreign keys.

Expected behaviour

There is a warning that people should be very careful about it, but it is allowed.