Add disabled field to User

www.agileplannerapp.com/boards/173808/cards/5886

This helps filter-out users based on their Signon state. For example, Signon would inform Publisher about a user being suspended by setting `disabled` in the user json.

Unpinning multi_json because the reason for which it was added seems to have been resolved.
I think this will still pick up the default value (it's defined on the model, not on the factory).  I think the only way to avoid this will be to perform a lower-level mongo operation after creating the user.