QR client setup

[wavexx]

It would be nice to allow to setup a client (such as PokeDL) just by using a QR code.
The QR code could be visible in the "Preferences" page in the user account, so that you can setup the client without external assistance.

The code itself needs to include all parameters: URL+user+password. The password though is not directly available to DL, as the password is hashed.

We could prompt the user to supply the password again before generating the code (which would be trivial), or support a different authentication scheme. Another approach would be simply to allow the user to generate a secret token to be used for the REST service only.