Fixes #16982 - Scope properly when no taxonomies are set

The default scope for hosts and other objects did not restrict
properly by taxonomies. An user without organizations or
locations, could do anything it's permissions allow to.
The list of hosts was unrestricted and showed hosts in...

Refs #15779 - make background processing unavailable for now (#4217)

The original PR got vetted in the packaging phase and including
foreman-task as dependency of Foreman was refused. We need to rethink
our approach for getting the foreman-tasks functionality available...

Fixes #15779 - make background processing available

fixes #16557 - move tests into modern Rails layout

Models, controllers, mailers and helpers are now in test/ subdirs
matching app/ subdirs. Service, miscellaneous and old test/lib/ tests
are now in test/unit/ separated from models, which obsoletes the custom...