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).
Change also adds minor feature to date-time selector so you can specify the time it uses as default when the value is 0 (disabled).
Credit: This feature was developed collaboratively by Charles Fulton, Neill Magill, and me.
Eloy Lafuente (stronk7)