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.

[skip sauce]
[skip variable]

[skip sauce]
[skip validator]

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

[skip sauce]
[skip validator]

[skip sauce]
[skip validator]

Refs #18150

[skip sauce]
[skip validator]

[ci skip]

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

Fix #17117

Fixes #17061