Reduce duplication between color/typography styleguide and SCSS configs.

[jgerigmeyer]

Currently the color and font names are duplicated between the styleguide (source/styleguide/styleguide.11tydata.yml) and the SCSS config files (source/assets/sass/config/color/_brand.scss and source/assets/sass/config/_typography.scss). It would be nice to have one single source of truth instead of manually keeping them up-to-date.