Caching overhaul

[johrstrom]

Caching throughout OnDemand is a bit wonky and it's been noticed that sometimes pages load a bit slow. This ticket is to start finding optimizations in what we can cache and what we can't. If we can cache something - we should. So this ticket, and the subtickets, should crawl all over OnDemand to find where we can optimize page loads. This is mostly for the dashboard, but could affect nginx_stage or apache.

• Dashboard: Processing erb on each request to generate the description information produces performance issues #210
• Configure NGINX to handle cache validation for static assets #6
• Dynamic pages being cached, could OOD send "no-cache" network headers? #2749