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

* Rename variable $input-border-focus to $input-border-color-focus
* Rename variables $pagination-hover-border, $pagination-active-border, $pagination-disabled-border to $pagination-hover-border-color, $pagination-active-border-color, $pagination-disabled-border-color, respectively
* Rename variables $state-*-border to $state-*-border-color, respectively
* Rename variables $alert-*-border to $alert-*-border-color
* Rename $list-group-active-border to $list-group-active-border-color
* Rename $table-inverse-border to $table-inverse-border-color
* Rename $btn-*-border to $btn-*-border-color
* Rename $navbar-*-toggler-border to $navbar-*-toggler-border-color

fixes #22007 and fixes #21976 by changing margin strategy for card deck gutters