WIP: Barcodes as private keys

This should let us:

1. print barcode on badges
2. use the barcode to decrypt user data at the kiosk: ergo kiosks will stay up even if the network goes down

TODO
* [ ] working symmetric encryption: I have not yet found an algorithm and configured it for perfect parity between the server and client.
* [ ] finish and test the JS input listener
* [ ] don't encrypt fields separately - that was just dumb

@sddhrthrt I have made you collaborator on this repo, so push any changes to the `barcodes` branch to make it appear under this PR