Make it easier to add payment methods for subscription autopay

Summary:
Fixes T7424. Ref T6308.

Currently, there's no option to just add a card directly from the autopay UI. Add a button so this works.

Also, chip away at T6308 a bit. This isn't perfect but looks a little less out of place.

Test Plan:

• Added a payment method, then set it as autopay.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6308, T7424

Differential Revision: https://secure.phabricator.com/D11935