Benefits layout; /enterprise, /ecommerce, /small-business, /publishers pages

@kylerush this creates a new benefits layout to be used for /enterprises, /eccommerce, /benefits, /publishers, and /small-business. Also creates /enterprises page which uses this new layout.

The sidebar form is a simple include that lives in new partials folder (website-guts/templates/partials)

Questions:
1. In the top nav menu ("Optimzely for:"), see how dropdown menu items change on different pages (e.g. https://www.optimizely.com/ecommerce and https://www.optimizely.com/enterprises). How do we want to handle this in the layout?
2. How should we handle layout-specific CSS and JS? Right now I'm including them on a per-page basis, but it would be nice if we could just put them in the YFM of the layout so child pages can inherit them.

Related issues: #11, #10, #14, #15, #16 
we may not need this span thing, but up to you when you refactor the html for the form