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.

Extraction order does not need to equal insertion order.

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

Thanks, Hypothesis.

Probably useful for implementation later.
Useful for test suite right now.

Helpful for the tests

This helps the client a lot with lease operations