Add a UI for payment methods

Another split from #2711013: Implement the initial payment functionality.

We need:
1) A UI that allows users to add payment methods
2) A default payment method add form, with appropriate styling
3) A form element for embedding payment gateway forms (such as the payment method add form)
4) Generic exceptions used for API requests (creating a payment method, creating a payment, etc)
5) Interfaces for payment gateways
6) The commerce_remote_id field on users, used to store the remote customer IDs.