Carefully consider balance when deleting an account

[novoselt]

Important starting point: for the logic below "balance" means exactly the balance in whole cents which is shown to the user, regardless of possible fractions and rounding.

• If the balance is positive, starting from $0.01, show a message to the user "You have $X.xx of CoCalc credit on your account. It has no cash value, but you will be emailed a CoCalc credit voucher for this amount valid for 5 years. You can apply this credit to any account you wish in the future or forward it to someone else."
• If the balance is negative and Stripe allows processing a charge for this amount, show a request for exactly that charge and a message "Once your payment is processed and your balance becomes non-negative, you can come back and delete your account."
• If the balance is negative, but too small for Stripe to process, consider forgiving it. As a safety check perhaps require some minimum amount of credit ever posted on this account.