MDL-24419 (4): Add conditional availability support to sections; section cache

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).

course/edit.php

@@ -126,6 +126,7 @@ if ($editform->is_cancelled()) {
         // Save any changes to the files used in the editor
         update_course($data, $editoroptions);
     }
+    rebuild_course_cache($course->id);
 
     switch ($returnto) {
         case 'category':