Allow getting default payment method, not just index
New signup: go to web to choose credit card This step, if chosen, directs the user to the jmp-pay webapp to add a credit card. It includes an OOB element for use by user agents that can handle that (possibly for webview embed, etc) with a note-based fallback as per XEP. Once the user chooses "next" the code checks if they have actually added a card, and if so continues to a yet-unimplemented step and if not repeats the instruction to go to the web app.
Split logic out into testable objects Rubocop clean. Good test coverage of helper objects.