Remove remaining traces of localStorage

Add window.storage to the background page, which loads all data from the
'items' store in indexeddb, caching them in memory for synchronous
access, then override textsecure storage to use that in memory store.

js/chromium.js

@@ -98,12 +98,12 @@
     window.textsecure = window.textsecure || {};
     window.textsecure.registration = {
         done: function () {
-            localStorage.setItem("chromiumRegistrationDone", "");
+            storage.put("chromiumRegistrationDone", "");
             extension.trigger('registration_done');
         },
 
         isDone: function () {
-            return localStorage.getItem("chromiumRegistrationDone") !== null;
+            return storage.get("chromiumRegistrationDone") === "";
         },
     };