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

eg: nix-build --arg trialArgs '["-x"]'

* switch from tahoe-lafs src on my laptop to github
* package required updated versions of tahoe-lafs dependencies
* use overlays to avoid version skew between different python libs
* copy/paste/edit the nixpkgs expressions instead of overrideAttrs because the
  latter doesn't converge, I guess?

Not currently working