Add disabled field on User

www.agileplannerapp.com/boards/173808/cards/5886

This helps filters 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.
Filter-out disabled users from lists and selects

www.agileplannerapp.com/boards/173808/cards/5886

Disabled users for now are suspended users in Signon.
They get synced over by [GDS::SSO](https://github.com/alphagov/gds-sso/pull/62).
Merge pull request #1855 from alphagov/ignore-disabled-users

Filter-out disabled users where users are listed