Service Worker improvements

[timroes]

The following improvements could still be made to the service-worker:

• Only download the required DPI avatars
• Rethink including all favicons sizes in precache
• Don't include history.html and add error handling for history load failure
• Add "offline available" and "new content" messages
• Don't cache piwik resources

Once the blog gets more and more posts, I must also rethink downloading all articles in advance, and perhaps only download articles within the same category of visited pages.