misc fixes:

- import from '@webrecorder/wabac' package for sw.js, though using github overrides for now
- replay: show loading spinner on beforeunload event in iframe
- embed: support passing basePageUrl param as part of extraConfig for custom top-level page

src/electron-preload.js

@@ -1,6 +1,6 @@
 "use strict";
 
-import { CollectionLoader } from 'wabac/src/loaders';
+import { CollectionLoader } from '@webrecorder/wabac/src/loaders';
 
 const { ipcRenderer, contextBridge } = require('electron');