Prevent unpublishing of CIPs.

This is a temporary measure until the relevant changes to the Unpublishing
model have been carried out.
Fix attachment preview on CIPs.

Template expects a sorted_organisations method on the edition. Added one which
simply returns the owning_organisation as an array.
Merge pull request #1461 from alphagov/fix_CIPs

CIPs: prevent unpublishing and fix attachment preview.