Make the files in the Ballista Cloud Demo persistent

[mgiuca]

Use IndexedDB to persist the virtual file system presented by the Ballista Cloud Demo.

Currently, all the state is stored in the foreground page's JavaScript context. By making it (locally) stateful, we can demonstrate the Ballista API's ability to receive updates while both the foreground page and service worker are dead. (Although in the latter case, our polyfill might not be able to do that...)