MDL-41114 Speeding up course listings on large sites

- function coursecat::make_categories_list() does not use retrieved contexts and has to query them again.
- /course/manage.php uses field coursecat:: that was not fetched when retrieving the course category children.

lib/coursecatlib.php

@@ -48,7 +48,7 @@ class coursecat implements renderable, cacheable_object, IteratorAggregate {
         'descriptionformat' => null, // not cached
         'parent' => array('pa', 0),
         'sortorder' => array('so', 0),
-        'coursecount' => null, // not cached
+        'coursecount' => array('cc', 0),
         'visible' => array('vi', 1),
         'visibleold' => null, // not cached
         'timemodified' => null, // not cached
@@ -1896,6 +1896,7 @@ class coursecat implements renderable, cacheable_object, IteratorAggregate {
             foreach ($rs as $record) {
                 // If the category's parent is not visible to the user, it is not visible as well.
                 if (!$record->parent || isset($baselist[$record->parent])) {
+                    context_helper::preload_from_record($record);
                     $context = context_coursecat::instance($record->id);
                     if (!$record->visible && !has_capability('moodle/category:viewhiddencategories', $context)) {
                         // No cap to view category, added to neither $baselist nor $thislist