Hide the sectors tag field for Whitehall artefacts

The canonical source of the topics (nèe specialist sectors) for a 
Whitehall artefact is Whitehall, therefore we should not allow these
to be edited in Panopticon as the changes are not propagated back to the
Whitehall editions.
Merge pull request #215 from alphagov/hide-sectors-editing-for-whitehall-artefacts

Hide the sectors tag field for Whitehall artefacts