[DRAFT] Fix the mess that is css

[TomNUSDS]

• Move scss source out of /assets/stylesheets/ and into the root /stylesheets
• Move the application.css (which is currently a static asset) into /assets/css
• Update build steps to READ from /stylesheets and output into /assets/css
• Update _head.html to use the /assets/css output location.
• Remove the SASS from _config.yml to test