Implement pairing process

[devinbileck]

For the PoC, we need a pairing process that is similar to that of the existing mobile app. This is done by sending the device token and encryption key from the mobile app to the Bisq client either by scanning a QR code generated by the app into the Bisq client or by emailing it from the app and entering it into the Bisq client.

The design work will be done in #29, so this will just need the back-end logic to generate the device token and encryption key, and generate/display a QR code.

See Pairing Process for more information.