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

See: https://github.com/twbs/bootstrap/pull/18255#issuecomment-237034763

Here we're doing some margin swapping, so it looks a little funky. All this does is match the margin implementation and rendering across our table and flex versions of card decks.

- Remove the box-shadow and switch back to border to match .card basics
- Update the header nav margin override--since we restored the border, we need that default negative margin at the bottom

- Drops the experiment I had going for `box-shadow`-powered borders
- Reinstates regular `border` using existing variables

Fixes  #19097 and #19143. Nullifies #19828.

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fixes #18940: use specific border properties when changing the corners of list groups in cards to avoid overriding combo first-/last-child items

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Refs #18150

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Changed card border radius image caps to use variable rather than static values

Fix #17117

Fixes #17061