Consider fingerprinting for JS/CSS assets for cache-busting

[patdavid]

Using Travis CI for the build and push process now means all the files for the site end up with new ETags (and there is no caching happening at the moment).

So we should be approaching cache options to keep the site snappy. Part of this might be to set long cache-control headers on JS/CSS and use fingerprinting to cache-bust on newness.

[patdavid]

Relevant
https://github.com/h5bp/server-configs-apache/blob/master/src/web_performance/expires_headers.conf