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 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.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6881

Differential Revision: https://secure.phabricator.com/D11596