WIP: Barcodes as private keys

[shashi]

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
• test printing tickets

@sddhrthrt I have made you collaborator on this repo, so push any changes to the barcodes branch to make it appear under this PR

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