Give Document formatters specific "kind"

Instead of being generic about how we register documents with panopticon give them an explicit "kind" which is stored with the PanopticonMapping as resource_type. In order to find these again we will need to change the PanopticonMapping#documents scope to pull all types of specialist-document.

While we are here lets also rename DocumentArtefactFormatter to a CmaCaseArtefactFormatter which is its correct name!
Remove legacy publishing for documents

Previously, we were doubling up on the contents of the details hash in
order to allow publishing to both the Finder API and Content API. This
commit removes the duplicated details hash.
Merge pull request #159 from alphagov/feature/bump-content-models-version

Allow new format types for specialist documents
Merge pull request #160 from alphagov/remove-legacy-dual-publishing

Remove legacy publishing for documents