Add self-checkout

[cbeer]

Fixes #299

TODO:

• tests
• some sort of confirmation that the barcode matches the book the patron expects (by retrieving the item bib info?)
• flash messages
• a button in the UI somewhere
• library policy changes?
• barcode scanner
• load quagga javascript only when necessary

Note that iOS requires the site be loaded over https in order to activate the camera... so use ngrok or something similar to test.

[jgreben]

Closing to reopen against branch: main.