QR-based UI for registration of multiple devices

[GallagherCommaJack]

Mobile adds desktop:

desktop: generate keypair, render public key to QR code
mobile: read QR code, sign public key, send signature to server

Desktop adds mobile:

desktop: generate keypair, sign public key, render keypair and signature to QR code
mobile: read QR code, check signature, write keys to disk, register with server

I expect we'll end up using these crates for QR-code stuff:

1. encoding: https://crates.io/crates/qrcodegen
2. decoding: https://crates.io/crates/quirs