Gold partners: add categories, support filtering

Adds categories to the gold partners, and a filtering bar at the top of the partners page:

![partner-categories](https://cloud.githubusercontent.com/assets/9033/3557045/2609ffca-092b-11e4-80df-c27aad59db65.png)

:eyeglasses: @bsimpson63 @umbrae @powerlanguage
:no_bell: If you wanted to get fancy here, this would probably be a better fit as a `nav` element. http://html5doctor.com/nav-element/

Not really necessary though. This content probably isn't a perfect fit for `section`, but I think it's gray enough that we want to just reuse the CSS we're OK here.