This removes the dependency on ``me.dm7.barcodescanner:zxing`` and replaces it
with our own QR code scanner implementation using CameraX and ZXing. The main
reason for this change is to hopefully get better compatibility with obscure
devices. The barcodescanner library we were previously using seems unmaintained,
while Google is apparently putting a lot of effort into CameraX.
ScannerActivity has been almost entirely rewritten, but the functionality is
exactly the same as before.
