Disable "Show online users" in "Administer institutions" if admin

disallows

Bug#976274

This drop-down menu will be disabled if site amin unchecks
"Show online users" in "Configue site"->"General settings"

Change-Id: I5f592787bfd6392b39c582ecba93be1270f66e14
Signed-off-by: Son Nguyen <ngson2000@yahoo.com>

htdocs/admin/users/institutions.php

@@ -398,6 +398,7 @@ if ($institution || $add) {
         );
         $elements['showonlineusers'] = array(
             'type'                  => 'select',
+            'disabled'              => get_config('showonlineuserssideblock') ? '' : 'disabled',
             'title'                 => get_string('showonlineusers', 'admin'),
             'description'           => get_string('showonlineusersdescription','admin'),
             'defaultvalue'          => $data->showonlineusers,