Bare-bones presenter for an Unpublishing

This deliberately skips a bunch of stuff that isn't needed for rendering an
unpublishing on GOV.UK
Skeleton view for unpublishings

For now, the controller now does a simple switch to use the correct presenter
and render the appropriate view for the content item that is loaded. Later, we
will more than likely extract this logic out somewhere else, but whilst we only
support these two formats, this is enough.
Raise an appropriate error for unsupported format
Merge pull request #13 from alphagov/unpublishing-format

Handle unpublishing format