Setup far-future expires headers for assets

This patch dramatically improves Foreman loading times. How to test:

* Enable developer console in Chrome or your browser
* Go to Hosts list and refresh clearing cache (Ctrl+Shift+R in Chrome)
* Note the loading time of main asset file (1.1 MB)
* Now click on the Host list link again
* Note it now says "Not modified" or "From cache" and loading is faster
  (with this patch applied)

For more info:

4.1.1 Far-future Expires Header

Precompiled assets exist on the filesystem and are served directly by your
web server. They do not have far-future headers by default, so to get the
benefit of fingerprinting you'll have to update your server configuration to
add them.

http://guides.rubyonrails.org/asset_pipeline.html#precompiling-assets