Don't allow debit for specific users

[cristan]

You're bound to run in to customers where paying is really troublesome. For this, you should be able to indicate they can't have any credit.

For this you'll need 2 features

Indicating that a user can't have any credit
This should be a regular boolean property of a customer.

Disallow ordering when applicable
After determining whether the user can order at all, we should now check if the user is allowed to have a debit, and when he/she doesn't, we should disable any products which would make this user have a credit below €0. This needs to be done every time a user is selected, a product is added or a product is deleted.

Checking server side
To prevent hacking attempts, we should not only check client side, but server side as well. If a user like this creates a transaction which would cause her/him to have a debit, the server should return an error.