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.
such was my intention, keen observation @alext ! corrected it.