build the first version of a "link devices" flow

given two devices:
1. on either device click "link device"
2. on the other device, scan the QR code or get the link to yourself SOMEHOW
3. one device delegates its account to the other's account url and then adopts that account url

unsolved questions:
- how do we decide which device's account URL should win? (low hash? age? number of files? user choice?)
- how do we update old references to the account URL attached to your now defunct account URL while minimizing hops and beanstalks?
- every other question