Use sass functions for box-shadow utilities

box-shadows do not lend themselves well to be "just" utility classes. We need some functions to make it easier to maintain shadows for use in components such as toggles, and buttons.

The following discussion from !876 (merged) should be addressed:

• @jboyson started a discussion: (+4 comments)

  @ealcantara can I get some feedback on how I'm proceeding with these? This is the duplicate "boilerplate" code I was talking about.