Added pagination to the items index.

I added pagination to the items index via will_paginate gem.  Default per_page is 12, but can be set in the url query params.  Appears to work both with and without a search term.  Added minimal styling to gss.css.scss -- not positive that's the correct place!
Let's restrict pagination to 12 per page. It's good to know we can make it configurable through a query param, but I don't think that's a feature we need.