Cancel subscriptions when an account is deleted

[novoselt]

To make sure that there are no attempts to charge for renewals. The license can stay as it until its expiration, in particular it may be providing upgrades to some collaborators.

Bonus - when a user with an active subscription/valid license/positive balance tries to delete the account, show an extra modal with a confirmation check box to the effect "I understand that I will permanently and irreversibly lose access to these". There may be a note that non-subscription licenses can be edited to end immediately for a refund to the balance and it is possible to create a voucher from the balance. (Our ToS explain, however, that purchases in CoCalc are not refundable except for errors, so no cash value.)