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 this app
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.
Merge pull request #249 from alphagov/ignore-disabled-users

Add disabled field to User