Merge branch 'new-signup-add-credit-card'

* new-signup-add-credit-card:
  Happy path for credit card signup
  Panic should work on any value for error
  Allow getting default payment method, not just index
  Object representing the backend SGX to use
  Stop polluting Object namespace with Blather DSL
  Work in the presence of em-synchrony
  Helper to allow ordering phone number from Bandwidth v2
  Helper to get a promise that resolves after N seconds
  Every payment kind will need the plan, so put it at the top
  Use registration pattern for Payment kinds
  Method to bill the plan of a Customer
  Use Forwardable for simple delegations
  Helper to allow using sync-style code in a Promise context
  New signup: go to web to choose credit card
  OOB helper
  Reject promise on stanza error
  Fix typo

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.