Fix question tests & editing for Qs with neither PRTs nor inputs #363

STACK has long has the feature to have 'questions' like Moodle's
descriptions, which neither inputs nor PRTs. however, this is not much
used, and seems ot have got a bit broken recently. This commit fixes all
the problems I found with a question like that.

This also stops the edit form from validating details of a PRT or Input
where the 'Confirm delete' checkbox has been checked, which I think is a
good change.