Initial registration flow for Bitcoin

This is the Bitcoin "happy path" where there is already a tel and a customer_id.
Gets all the way to generating a BTC address for payment and quoting an amount.
Stubs out flows for credit card and activation code as well, but does not
implement those flows.

TBD: after bitcoin payment comes in and jmp-pay activates the account, we should
come back here to actually buy the number, select a new one if no longer
available, etc.

Split logic out into testable objects

Rubocop clean. Good test coverage of helper objects.