R1:9c4302927760

Genericize "Add Payment Method" form

Summary:
Ref T2787. For payment methods that allow you to add a billable method (i.e., a credit card), move all the logic into the provider. In particular:

- Providers may (Stripe, Balanced) or may not (Paypal, MtGox) allow you to add rebillable payment methods. Providers which don't allow rebillable methods will appear at checkout instead and we'll just invoice you every month if you don't use a rebillable method.
- Providers which permit creation of rebillable…