Not in the database but in the Python and JSON representations

And a corresponding `created` field to the voucher model object which
propagates to the web view.

Turns out, it did already.  Make the use order explicit in the model, anyway.

lots of mess to clean up though

Also more docstrings
Also better test coverage of the non-urlsafe-base64 error case

* flexible schema requirements
* in-memory testing
* improved error checking in the view