Drop `color()`, `theme-color()` & `gray()` functions

Created by: MartijnCuppens

Drop color(), theme-color() & gray() functions in favor of variables. The functions just called a map-get() of a map where just the variables were defined.

Also the theme-color-level() now accepts any color you want instead of only $theme-colors colors. The first value now is a variable instead of the $theme-colors key.

TODO:

• Document breaking change in migration guide
• Rename theme-color-level() to color-level()

https://deploy-preview-29083--twbs-bootstrap.netlify.com/docs/4.3/getting-started/theming/