Service Worker: add support for Cache expiration

In `Offline Mode`, we could take advantage of the sw-toolbox [Cache Expiration](http://googlechrome.github.io/sw-toolbox/docs/releases/v3.2.0/cache-expiration-options/) feature to limit how long story and comment threads get offline cached.

This is useful if you've been using the app for a few weeks, have built up a large offline cache of content and would prefer that we only keep stories/comments stored for a week or something like that.