[feature request] ability to know when a subscription will renew

[andheiberg]

It is in some cases common curtesy to let a customer know that something will be renewing and allowing them to cancel. There are currently know way of knowing when to do this.

The time of subscription added to the billing table should be sufficient. Granted it would be the nicest to have a billing_next_payment_at column that updates, but this does not seem easy to implement.