Clay CSS import Bootstrap 4 components independently for `base.scss` and `atlas.scss`

[pat270]

Create a new Sass partial _bs4.scss or equivalent to import Bootstrap 4's components instead of relying on bootstrap/bootstrap.scss. This allows us to remove Bootstrap 4 imports as we absorb them into Clay CSS. This also allows bootstrap.scss to remain intact for those who use it.

See base.scss and atlas.scss and related to #3149