Fingerprint assets

[mermop]

We should fingerprint assets (index.css and index.js in particular) to prevent overzealous caching messing with deploys

[ndorfin]

This will be particularly useful for when we create Service Worker caches.

It could be as easy as enabling asset hashing in Middleman.

There are some files we don't want to asset-hash, such as:

opengraph.png
*touch-icon*.*
service-worker.js
*.xml
*.txt
*.json
favicon.ico

These files should be listed in middleman's asset-hash's ignore param.

We'll still need a separate task to set up CloudFlare invalidation in config.rb