Fetch account should require a device signature

[tmpfs]

Otherwise revoking a device is not effective as a revoked device could still just fetch all the account data at a later time using just the account key.

To make this possible the device sending the server and private key would need to trust the public key of the other device.

There is an interesting cross-platform bluetooth implementation here:

https://github.com/deviceplug/btleplug

But a quick test yields an abort on MacOS so possibly the better way to communicate between the devices would be an end-to-end encrypted tunnel over the server.

[tmpfs]

There is a fix for the abort, described here: deviceplug/btleplug#106 (comment)