refactor: clean up the default app implementation (#14719)

* Disable nodeIntegration
* Enable contextIsolation
* Re-implement the CSP security check to handle running in
contextIsolation
* Disable bad DCHECKS for the promise helper
* Remove the unused "-d" flag for the electron binary
* Added a way to hide the default help output for electron devs who
don't want to see it every time

default_app/index.html

@@ -83,8 +83,6 @@
       </div>
     </div>
   </nav>
-
-  <script src="./renderer.js"></script>
 </body>
 
 </html>