MDL-68148 core_user: set the fullname column as the header column

ref: https://www.w3.org/WAI/tutorials/tables/

user/classes/participants_table.php

@@ -220,6 +220,9 @@ class participants_table extends \table_sql {
         $this->define_columns($columns);
         $this->define_headers($headers);
 
+        // The name column is a header.
+        $this->define_header_column('fullname');
+
         // Make this table sorted by last name by default.
         $this->sortable(true, 'lastname');