Variable font assets

[tylersticka]

Although our efforts to add Source Sans Variable to the current iteration have been somewhat frustrating (as documented in #452, which you should totally read before starting this). A major release gives us an opportunity to revise our strategy for this.

To keep us from stalling completely, I will be adding a separate issue for font loading strategies. This issue should prioritize getting it working in a simple way that sets us up for future success.

Tasks

• Generate/procure/include necessary font files
  • Source Sans Variable (body copy)
    • WOFF2 / Regular
    • WOFF2 / Italic
  • Source Sans Pro (fallback where variable fonts aren't supported)
    • WOFF2
      • Regular
      • Semibold
      • Bold
      • Italic
    • WOFF
      • Regular
      • Semibold
      • Bold
      • Italic
  • Source Sans Code (code samples)
    • WOFF2 / Regular
    • WOFF / Regular
• Add CSS/Sass necessary for usage