Implements hook_preprocess_HOOK() for maintenance-page.html.twig.
function seven_preprocess_maintenance_page(&$vars) {
// While markup for normal pages is split into page.tpl.php and html.tpl.php,
// the markup for the maintenance page is all in the single
// maintenance-page.html.twig template. So, to have what's done in
// seven_preprocess_html() also happen on the maintenance page, it has to be
// called here.
seven_preprocess_html($vars);
}