Add configurable settings for Stripe

The new model, StripeCreditCardSettings, is intended to replace the
existing CreditCardSettings which was created specifically for First Data.
Hook up StripeCreditCardSettings module extension
Add a function to check setup of credit card module.

This is mainly for admins, who may add the credit card module to a program
and not know that any more setup needs to be done.

More setup does need to be done, since we can't publicly distribute Stripe
account keys.  However, we can put keys in local_settings.py on the
production servers, in which case admins won't need to think about it.
Add a function to send an e-mail if there is a credit card error
Handle successful payments via iac.submit_payment()
Add handling of various credit card exceptions

Credit card payments require more attention to testing and exception
handling.  I have added checks for the various error codes you can get
from Stripe (and tested them by supplying the credit card numbers that
Stripe has reserved for checking this).

The template the user sees gives them some hint of what kind of error
occurred, in case this might be reassuring.  (Full details are e-mailed to
the program director and websupport.)
Clean up payment page template and add explicit line item for donation
Fix return links on credit card success page
Rename files for credit card module to Stripe
Rename credit card module class to Stripe
Install new module during migration
Remove old code which was specific to First Data
Fix a couple of minor template issues