Marketing site: Configure SCSS, PostCSS, and SVG Sprites

As per larrybotha/mooc-floss#1 (closed) this MR has the following:

• SCSS
• PostCSS for:
  • autoprefixing CSS values for legacy browsers
  • support for CSS logical properties
  • support for CSS custom properties
• SVG Sprite
  • automatic building of an SVG Sprite from individual svg files when those files change

Additionally, it resolves an issue in the pre-push hook where accessibility tests were not being run due to a dependency returning a non-zero exit after tests complete successfully.

This MR was branched off of the branch in !42 (closed).