Test fix: verify payments are successful.

And it turns out they aren't! Not because of any real-world problems
with the code, but because testing creates duplicate transactions, which
Authorize.Net flags. So now we basically tell ANet to ignore duplicates.
Removed useless diagnostic logging.

It would be useful if the billing code used validations to help users
understand why their payments fail, but oh well!
Have Authorize.Net send receipts to customers.

... by supplying an email address, which we have to collect in the
billing form.
Set email for provisionary access payment.
Change quote on homepage

https://www.pivotaltracker.com/story/show/52222761