switch to rgba for list-group border color

renamed modal-title-padding to modal-header-padding

Refs https://bugs.chromium.org/p/chromium/issues/detail?id=510079
Closes #21120

[skip sauce]

Feature/omit xs from grid

Update visual tests.

Change .tag print styles to .badge.

Fix padding on .bg-faded demo.

it's → its

Small grammar fix.

Fix MS A11y UserVoice entry in Most Wanted Features list

And move one Firefox entry to keep the entries sorted by browser.
Refs #21089

Refs #21089
[ci skip]

Linked to slack

Fix broken and redirected links.

Scope ::first-letter selector for print styles

`*::first-letter` causes problems in IE11 - only prints first page of a
multi-page document

Add Safari skip link bug and Firefox/Edge AT focus feature requests

These would help in making hidden skip links show up visually once they
receive AT focus (for sighted/partially sighted AT users)

The new breakpiont-limited class pattern is to omit the breakpoint size when using the lowest size eg. col-xs-12 to col-12. This commit implements this pattern to the grid system.

* Explore responsive display utils, but with a twist: lowest breakpoint has no breakpoint modifier in the class name
* make floats use the same format, add float-none mixin

* make spacer utils responsive by grid tier
* update scale to add two levels, document them
* change responsive spacing utils to avoid the xs abbreviation in the class name for that tier
* update code snippet to match source
* update usage in our docs
* linter
* docs updates