LP#1622696 Webstaff credit card payment support

LP#1622696 Webstaff credit card payment support

Support collecting credit card info for both EG-processed and
externally-processed credit card payments, consistent with the XUL
client.

Refactor the chain of events leading up to payment submission,
including verifying warning-level payment amounts, annotating payments,
and collecting credit card data so the user is able to perform any or
all of these steps and cleanly cancel at any point along the way.

Signed-off-by: Bill Erickson <berickxx@gmail.com>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>