Move forays, courses, and workshops up a level

Rather than have everything nested under `learn`
we want to keep our URLs short. We can still have
a learn index that points to these sections at 
some point.
We don't need "global" prefix for scss partials
Reorder learn index links
Refine layouts

Use nested layouts and partials to reduce 
redundancy.

* Add site layout for basic HTML used by default
* Add home layout for home page
* Add navigation partial
* Add currently partial for homepage content
* Remove default index page content for home
Stylin