Filter-out disabled users where users are listed

www.agileplannerapp.com/boards/173808/cards/5886

Adds a `disabled` field on User, which is intended to reflect Signon user state. For example, Signon would inform this app about a user being suspended by setting `disabled` in the user json. see related [GDS::SSO change](https://github.com/alphagov/gds-sso/pull/62).

This allows us to skip suspended users from the 'Author' drop-down on the documents list page, and remove their names from the users master list in admin.