Trying to find a new way to do responsive card decks while not locking ourselves into the cards themselves. My thinking here is we can easily control the column (.col) width by the parent, but I don't know how many we need (have 0-5 now) across each breakpoint. This works for cards so far, and I think could get us equal height, too.

Co-Authored-By: Mark Otto <otto@github.com>

Previously deprecated in v4.x, this clears out the now unused Sass option and removes some unused mixins. Arguably we could remove more, but I like the hover-focus mixin and we make extensive use of it across the project.

Define new variable for card header color

Uses orphans/widows trick from @fran-worley at https://github.com/twbs/bootstrap/issues/20925#issuecomment-333492739.

Fixes #20925.

1. Instead of having two selectors, we use the `.list-group-item` class and have coverage for list items and anchors.
2. Rather than override all border values, we only override what we set with `border-top`.

remove - vars, update docs to generate from data yml files, update mixins, and generate card, button, bg, and color classes from the sass map