R1:77db15c47bd7

Automatically bill subscriptions when a payment method is available

Summary:
Ref T6881.

- Allow users to set a default payment method for a subscription, which we'll try to autobill (not all payment methods are autobillable, so we can't require this in the general case, and a charge might fail anyway).
- If a subscription has an autopay method, try to automatically bill it.
- Otherwise, we'll send them an email like "hey here's a bill, it couldn't autopay for some reasons, go pay it and fix those…