Expose voucher state via the web interface

Not in the database but in the Python and JSON representations

Track and expose more Voucher metadata: creation time and redeemed token count.

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

Finish the web interface for requesting all unused tokens

Extraction order does not need to equal insertion order.

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

Add a web interface for retrieving the number of unblinded tokens remaining.

Unblinded tokens have a one-to-one correspondence with ZKAPs.  That is, the number of ZKAPs
remaining is equal to the number of unblinded tokens remaining.  ZKAPs are created at the point
of use, though, so they cannot be counted/listed/exported/etc directly.

Thanks, Hypothesis.

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