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

* fix image stretching due to flexbox
* fix broke text-muted on dark bg
* no img-fluid needed

- Instead of negative left/right margins, we selectively apply margins to the cards as needed. This way the first and last child never receive a left and right margin (respectively), so we don't need to negative indent those at the .card-deck level.

- Drops the margin-bottom override on the .card because there's no more default margin there.

- Drop the margin-bottom from the .card-deck to match our lack of margins on the card.

This and the previous commit fixes #19883.

add flex direction too

use utils instead

this allows us to easily scale card blocks in groups and decks, without repeating it in our css

this assumes no margin on the .card to start (which will follow in a later commit)

* remove the $enable-flex variable option

* remove bootstrap-flex.css dist file and it's grunt task

* remove the separate flex css file for docs; it's all the same now

* remove flexbox docs (porting some to the main grid docs in next commit)

* clean up few grid docs bits to simplify copy, start to mention flexbox

* port relevant flexbox-grid.md content to grid.md

- clean up mixins
- update how it works section
- bring over sizing and alignment sections

* remove the $enable-flex from the options.md page

* update lead paragraph to mention flexbox

* update migration to mention loss of ie9 support

* remove mention of flexbox dist file

* clarify IE support

* making a note

* remove flexbox variant mentions from component docs

- updates docs for media object, navs, list group, and cards to consolidate docs
- no more need to callout flexbox variants since it's now the default

* remove $enable-flex if/else from sass files

* remove flex dist files

* update scss lint property order to account for flex properties

* linting

* change to numberless classes for autosizing, wrap in highlighting div

* bump gruntfile and postcss to ie10

* redo intro sections

* rearrange

* phew, redo hella grid docs

- rearrange all the things
- consolidate some bits

* remove reference to flexbox mode

* more border action for demo

* Make some changes to the .card's in .card-deck's to ensure footers align to the bottom

* Remove comment that duplicated some code
* Use transition mixin whenever possible
* Create a new function to reduce duplication
* Use the new `breakpoint-infix` method

* Have same margin for <hN> as a <div> on .card-headers
Related #18726

When you use a `<hN>` as `.card-header` there is `margin-bottom: .5rem` added. The changes set the margin-bottom to 0
* Removed space

* Add .img-fluid to card images in columns to ensure proper resizing
* change that variable from regular value to variable
* use calc to figure out a tighter border for card images

Cards must be inline-block in columns, otherwise they can get split across columns