MDL-81481 assign grading: speed up assign 'View all submissions' page

the 'View all submissions' page was fairly slow when dealing with large databases.

Solution: Utilize a static 'capability_checker' in 'filter_user_list' to prevent reinitialization.
 Requests made by 'get_users_by_capability' will be cached, thus enhancing performance for large databases.

availability/classes/info.php

@@ -634,7 +634,12 @@ public function filter_user_list(array $users) {
             return $users;
         }
         $tree = $this->get_availability_tree();
-        $checker = new capability_checker($this->get_context());
+        static $checkers = [];
+        $context = $this->get_context();
+        if (!array_key_exists($context->id, $checkers)) {
+            $checkers[$context->id] = new capability_checker($context);
+        }
+        $checker = $checkers[$context->id];
 
         // Filter using availability tree.
         $this->modinfo = get_fast_modinfo($this->get_course());