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. 
+1 on this being a constant. Or alternatively a simple utility function that takes the username and returns it with the prefix.

I actually prefer the second solution in case we end up doing more username changes as juju updates its stuff, but would be satisfied by this being defined as a const somewhere.