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.