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 methods handle their own data entry, since this will be per-provider.
• "Add Payment Method" now prompts you to choose a provider. This is super ugly and barely-usable for the moment. When there's only one choice, we'll auto-select it in the future.

Test Plan: Added new Stripe payment methods; hit all the Stripe errors.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D5756