R1:7a5f62282017

General cleanup for adding payment methods in Phortune

Summary:
This has no real behavioral changes (except better error handling), it just factors things out to be a bit cleaner. In particular:

- Move more shared form behaviors into the common JS form component.
- Move more error handling into shared pathways.
- Make the specialized Stripe / Balanced methods do less work.

This needs some more polish for nontrival errors (especially on the Balanced side) but none of the error behavior is worse…