Rename "payment reference number" to be "vouchers"

"payment reference number" has a number of shortcomings as a name for the concept it is attached to.

• The value itself is not really numeric. Canonically, it's an opaque byte string.
• It may be desirable to issue these when no payment has actually taken place (eg, trial access)

Discussion of these issues and the nature of the value in question led to the idea of "voucher" as the correct word.