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 if you want". - (That email doesn't exist yet but there's a comment about it.) - Also some UI cleanup. Test Plan: - Used `bin/phortune invoice` to autobill myself some fake test money. {F279416} Reviewers: btrahan Reviewed By: btrahan Subscribers: epriestley Maniphest Tasks: T6881 Differential Revision: https://secure.phabricator.com/D11596