Reservations for payment failures

[jace]

When a customer's payment fails, they are often not able to attempt another payment until the refund is processed, especially when it's a debit instrument and the money is no longer in their bank account.

Boxoffice should offer event organisers (and customer service staff) a way to reserve a ticket at the attempted price for about 10 days, until the customer has money again. Options:

1. Automatically mail this code to the buyer, assuring them we'll hold the price (until code expiry, or tickets being sold out). However, this risks abuse, as buyers aware of this scheme will intentionally cancel a payment while they make up their mind about paying.

2. Let a customer service representative offer this code when the customer asks for help. However, this risks losing those who don't ask for help. Our data should help us find the ratio of failed payment buyers who write in.

FreshMenu had a variant of the option 1 hack. Abandoning a cart almost guarantees a 15% discount coupon being sent in a short while. Since FreshMenu sets menus for the entire day, doing this in the morning will result in a discounted meal by mealtime. (FreshMenu rate-limits the availability of this discount to prevent abuse, but it does work the first few times at least.)

[jace]

Related: #128 (automatic re-processing). When that fails, the code should be generated.