Use first byte of QR code payload to indicate format version

The QR code payload currently includes the format version as part of a BDF list, which is base64 encoded on the beta branch but not on the master branch. This makes it awkward to detect incompatible versions.

Move the format version to the first byte of the payload and reserve the byte that's at the start of base64-encoded BDF lists so it's never used as a format version. This will allow us to detect incompatible versions by checking the first byte without having to try different encodings.