storage.js: Use substring instead of startsWith on strings

IE11 doesn't support startsWith..

public/js/icinga/storage.js

@@ -37,7 +37,7 @@
      */
     window.addEventListener('storage', function(event) {
         var url = icinga.utils.parseUrl(event.url);
-        if (! url.path.startsWith(icinga.config.baseUrl)) {
+        if (! url.path.substring(0, icinga.config.baseUrl.length) === icinga.config.baseUrl) {
             // A localStorage is shared between all paths on the same origin.
             // So we need to make sure it's us who made a change.
             return;