Add the user- prefix to the users username for all external requests

Any time the GUI needs to talk to juju-core it needs the 'user-' prefix to the username which we don't require the user to input when logging in. 
So to be more clear, I'd like the prefix to be added and removed only in one place, and I suspect this place is close to the login form. Note that the logic above, if required, would fail for a user named "user-user-foo".