Use localStorage & app cache

Issue Summary

Now that the CDN module's Far Future expiration support has landed (#974350: Far Future setting for Origin Pull mode), we can do something very cool: use localStorage and app cache to enforce even longer caching.

Browsers that support LS (localStorage) & AC (app cache) don't tend to flush those. The "regular browser cache", which is the one that's being used thanks to Far Future expiration, *does* get flushed relatively frequently (and depending on the device). Since it is now easy to track which resources can be cached permanently (any with a "cdn/farfuture/" substring), it is also fairly easy to leverage these caches.

This will especially make repeat visits on mobile devices significantly faster.