Offline recipes

Add a way to store recipes offline for access later, probably using LocalStorage and HTML5 offline cache manifest to show a list of available offline recipes and the fact that the site is operating in offline mode. When getting online again the offline cache should be updated.

The issue names LocalStorage and the HTML5 offline cache manifest as the technologies to investigate; start by locating the current recipe display and the browser entry point. Done means users can see available offline recipes and an offline-mode indicator, with the offline cache updating after connectivity returns.

Written by the indexing model from the issue text.