Minor refactor of doPayment, extracting to separate functions. NFC.

The primary aim of this refactor was to remove the http redirect(),
since this means the doPayment function could not be used for embedded
checkout in InlayPay. I also tried to split functions so that each did a
specific thing, and in particular, to separate out the munging of the
payment_params array from the doing-things-with-stripe code. I hope this
will make things clearer going forward and more easily integrated in
other APIs, like InlayPay.