The main issue to fix is that questions vesions which should not have
been used (that is, hidden or draft versions) were getting offered
as an option and acutally being used.

As part of this I was able to substantially un-tangle
mod_quiz\question\bank\qbank_helper, which previously was
a mass of functions calling other functions in a complicated way.
Hopefully, it is now a bit easer to understand, and perhaps
less buggy.