Fixed the homepage so it doesn't look messy on phones.

https://www.pivotaltracker.com/projects/646869#

Basically the "hero unit" was rendering horribly on very small screens
(eg. phones).  I've made it so that the hero unit is only shown on
"desktop" and "tablet" devices, not "phone".

I also moved the actual content of the hero-unit out into a partial to
avoid repeating myself.