MDL-64383 user: Make username required

Username is a required field, we should indicate this even if it has a help icon.

user/editadvanced_form.php

@@ -99,6 +99,7 @@ class user_editadvanced_form extends moodleform {
 
         $purpose = user_edit_map_field_purpose($userid, 'username');
         $mform->addElement('text', 'username', get_string('username'), 'size="20"' . $purpose);
+        $mform->addRule('username', get_string('required'), 'required', null, 'client');
         $mform->addHelpButton('username', 'username', 'auth');
         $mform->setType('username', PARAM_RAW);