Add user.devices related name
Refactor http-sig based authentication.

I needed to refactor and remove the djangorestframework-httpsig module
as it verified the signatures in a non-standard way and actually
bypassed the rsa-based verification.

 * We now have proper private/public keys
 * Authentication is now targeted to a specific device
 * we are now verifying rsa-keys properly
 * We are now verifying the http-host header properly