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 if you want".
- (That email doesn't exist yet but there's a comment about it.)
- Also some UI cleanup.
