And subsite change detection

Closes silverstripe#515

client/javascript/LeftAndMain_Subsites.js

@@ -3,8 +3,33 @@
 (function($) {
 	'use strict';
 	$.entwine('ss', function($) {
+
+        /**
+         * Store current subsite id and ask to reload the page if it detects any change
+         */
+        function detectSubsiteChange(selectedId) {
+          var sessionKey = "admin_subsite_id";
+          try {
+            localStorage.setItem(sessionKey, selectedId);
+
+            window.addEventListener("storage", function () {
+              var tabId = localStorage.getItem(sessionKey);
+              if (tabId && selectedId != tabId) {
+                var msg = ss.i18n._t("Admin.SUBSITECHANGED", "You've changed subsite in another tab, do you want to reload the page?");
+                if (confirm(msg)) {
+                  window.location.reload();
+                }
+              }
+            });
+          } catch (e) {
+            // Maybe storage is full or not available, disable this feature and ignore error
+          }
+        }
 
 		$('#SubsitesSelect').entwine({
+            onmatch: function () {
+              detectSubsiteChange(this.find("option[selected]").attr("value"));
+            },
 			onadd:function(){
 				this.on('change', function(){
 					window.location.search=$.query.set('SubsiteID', $(this).val());