Allow unpublishing of invalid editions

We want to allow legacy detailed guides that don't have any user needs
to be unpublished without having to enter one. I *think* this is a
good idea, but would love some feedback.
Merge pull request #1033 from alphagov/59226824-unpublishing-detailed-guides

Unpublishing detailed guides

https://www.pivotaltracker.com/story/show/59226824