Credit: original version done by Kirill Astashov of NetSpot (netspot.com.au),
finished and tweaked by sam.
This change adds conditional availability support for sections analagous to
that already available for activities. (Backend, UI, backup/restore.)
In order that this feature does not reduce performance, section cacheing has
also been added using a new course 'sectioncache' field analagous to modinfo.
The new feature integrates with activity availability so that activities
inside sections which are not available are automatically not available
themselves (meaning it works to restrict access).
By the way, the current style of creating course_sections is ugly. We
should probably have something like
make_sure_that_at_least_default_section_exists($courseid, $sectionid=null)
and not to insert it into database at many places as we do now.
Eloy Lafuente (stronk7)