Filter-out suspended user from user lists

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

Uses `disabled` on `User` introduced by the govuk_content_models
bump-up, to skip suspended users from:

* the 'Assigned to' drop-down on the editions edit
* the 'Filter by users' search in admin
* the 'Assignee' drop-down in on the editions filter view
Merge pull request #304 from alphagov/ignore-suspended-users

Filter-out suspended user from user lists