Consider using calcite-colors instead of defining them in our Sass

We re-define the [calcite-colors](https://github.com/Esri/calcite-colors) in our [Sass](https://github.com/Esri/calcite-bootstrap/blob/master/lib/sass/calcite/_colors-default.scss). It might make more sense to pull in the calcite-colors repo and borrow those color vars as they could change in the future and it might be more maintainable. We could do the same with calcite-icons when it is ready.

Thoughts?

@TheBlueDog

Start by reviewing lib/sass/calcite/_colors-default.scss and the linked calcite-colors repository to compare the existing definitions with the external variables. Check how Sass dependencies are currently pulled into calcite-bootstrap and whether calcite-icons is in scope. Done means the duplicated color definitions are replaced with a maintainable dependency without changing the theme's expected colors.

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