Use .scss instead of .sass

[wojtek-krysiak]

Use .scss instead of .sass

• It's still SASS, just a modern, more familiar version of SASS which makes contributing easier. Also, CSS is valid SCSS.
• Using @extend to create base styles from which other styles can extend. This results in a performant stylesheet that renders all shared styles in single declaration blocks, and aberrant styles, i.e., one-off margin adjustments, etc. to their respective component blocks.
• Adding typography.scss
  • All font-related styles integrated into a single place
• removing third-party (vendor) CSS and use npm to ease the upgrade process for those vendor styles, e.g., prettify styles.

[JustinBeaudry]

I've got this one done as well. Just need to make a PR. Can you assign this to me?