Fixes 2 bugs:
1. All keydowns were being prevented. Because of that the user wasn't able to navigate in the whole page using ARROW_UP/ARROW_DOWN.
2. Even when is an input or textarea the keydowns were being prevented. Because of that the user wasn't able to type any text on these elements.
- better copy and examples throughout
- callout on flexbug #12 for inline elements like links and images (which is the problem in #20408)
- add order section
Sets minimum required flex styles (including an explicit starting alignment) and removes everything else.
- no need for .media-left and .media-right, we have padding utils
- no need for a .media-heading, we have margin
- no need for .media-object, we have display utils
- no need for .media-list, we have .list-unstyled util
- 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.
- Drops the absolute positioning of the icons within the left/right controls. We have to keep the controls themselves positioned though since we're overlapping HTML elements here.
- No more position, left, right, or margins involved; just some justify-content and align-items.
- Add some comments for explaining which flex property-value pair does what.
- Remove the unapplied font and line-height stuff now that we're all SVGs and flexbox here.
This revamps the indicators to use flexbox instead of inline-block for added flexbility (hah). Indicators now automatically scale based on the number of elements present, and max out at the `$carousel-indicator-width` instead of always being that wide.
