Hide the sectors tag field for Whitehall artefacts

https://www.pivotaltracker.com/story/show/79283798

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.

We're addressing this issue now as we're now going to be registering Whitehall content directly with the content store in addition to Panopticon, so the need for a single place where Whitehall editions can be tagged with topics is important.
Would be good to indent this.

Also, would be good to have a test that removing this field from the form doesn't cause any existing specialist sectors on an artefact to be lost if the artefact is edited.