Set special header for 503 maintenance mode

This removes ambiguity with a 503 returned by app code, web server or
similar. Front-end and clients can then handle this state accordingly.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

lib/base.php

@@ -295,6 +295,7 @@ public static function checkMaintenanceMode(\OC\SystemConfig $systemConfig) {
 		if (((bool) $systemConfig->getValue('maintenance', false)) && OC::$SUBURI != '/core/ajax/update.php') {
 			// send http status 503
 			http_response_code(503);
+			header('Nextcloud-Maintenance-Mode: 1');
 			header('Retry-After: 120');
 
 			// render error page