Bdzim backup restore

This backup will include encryption keys and passwords and so needs to be zero knowledge. The critical part of the code is in bin/update_backup.py. That's the code that encrypts the backup and sends it to the accounts api. 
I don't want to block this merge because of this, but just for the sake of clarity: this is the sha256 import that doesn't seem to be used, which made me wonder whether pep8 was being used.

Obviously it's not a really problematic issue.