Add Sass `tint` and `shade` functions to fix/enhance the main navigation.

[adamjohnson]

This issue is a copy of wvuweb/wvu-design-system#293.

Examples

You can see a site without these functions (ugly borders):

• https://academicintegrity.wvu.edu/

And a site with these functions (nice borders):

• https://oedc.wvu.edu/

A full breakdown of this issue + instructions on how to fix this issue are linked above. I can submit a PR next week (hopefully).