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 
@acreighton do you think we need this? i'm thinking that this layout will only be used on "optimizely for" pages so i'm not certain we want to add another page level yfm property. i'd like to try and prevent adding more page level yfm properties as much as possible. what do you think?