Use non-deprecated call to activate navigation entry

OCP\User::checkLoggedIn();

OCP\App::checkAppEnabled('rainloop');

OCP\App::setActiveNavigationEntry('rainloop_index');

\OC::$server->getNavigationManager()->setActiveEntry('rainloop_index');

// Load the empty file ../css/style.css, that's needed to allow theming of

// the Nextcloud header and navigation if rainloop is the active app.