MDL-48660 Availability: filter_user_list() should respect 'view hidden'

Updated filter_user_list to account for the viewhiddenactivities and
viewhiddensections capabilities.

availability/classes/info_section.php

@@ -56,6 +56,10 @@ class info_section extends info {
         return \context_course::instance($this->get_course()->id);
     }
 
+    protected function get_view_hidden_capability() {
+        return 'moodle/course:viewhiddensections';
+    }
+
     protected function set_in_database($availability) {
         global $DB;
         $DB->set_field('course_sections', 'availability', $availability,