Whitehall support

Add a form that allows content editors to manually create artefacts for content in Whitehall. This allows Whitehall artefacts to be added to browse pages, and to a mainstream artefact's related items.

The secondary purpose is that it moves us towards all Whitehall content being registered in Panopticon, but that is some way off yet.

The reasons for having a separate form are that the behaviours for editing Whitehall artefacts are different - for example we need to be able to edit the name and description, even when published, as Whitehall won't update them for us, in the way that eg Publisher does. There are also many fields that just don't apply.