Get involved page

This is a clone of Kelli's "Get involved" poster, drawn up in HTML and CSS as faithfully as possible. 
Uses a sprites image file to reduce image GET calls.
Includes CSS to make this weakly responsive, or at least readable on sub-optimal monitors.

@brotatos This is my final significant contribution to the site, so I recommend you merge very soon. 

Fixes #204 
Fixes #246