Crib SW Gadget: Crib Enable Gadget is defined by local variable

* Effectively this allows to edit two Sites at the same time in two
differents tabs
* It also to not change the site you are editing in a tab by changing it
in another tab

gadget/crib-sw-gadget.js

@@ -3,12 +3,21 @@
 (function (window, rJS, RSVP, jIO) {
   "use strict";
 
-  function getStorageGadget(gadget) {
+  function getStorageGadget(gadget, new_url) {
     var storage_gadget, site_editor_gadget_url, getURL = window.location;
-    return gadget.getSetting(
-            "site_editor_gadget_url",
-             getURL.protocol + "//" + getURL.host + "/crib-enable.html"
-          )
+    return RSVP.Queue()
+      .push(function () {
+        if (new_url) {
+          return new_url;
+        } else if (!gadget.props.storage_gadget_url) {
+          return gadget.getSetting(
+                "site_editor_gadget_url",
+                 getURL.protocol + "//" + getURL.host + "/crib-enable.html"
+              );
+        } else {
+          return gadget.props.storage_gadget_url;
+        }
+      })
       .push(function (url) {
         if (gadget.props.storage_gadget_url == url) {
           return gadget.getDeclaredGadget("storage")
@@ -54,7 +63,7 @@
     try {
       new URL(string);
     } catch (_) {
-      return false
+      return false;
     }
     return true;
   }
@@ -151,14 +160,14 @@
           // run accidently in a possible bug in RenderJS when the same gadget
           // is declared twice in parallel. CLN needs to reproduce and
           // investigate this issue.
-          return getStorageGadget(gadget);
+          return getStorageGadget(gadget, url);
         });
     })
     .declareMethod('getCribEnableGadgetUrl', function (url) {
       var gadget = this;
       return new RSVP.Queue()
         .push(function () {
-          return gadget.getSetting("site_editor_gadget_url");
+          return gadget.props.storage_gadget_url;
         });
     });