Add Embed responsive utility
Merge branch 'responsive-embed' of https://github.com/boulox/bootstrap into boulox-responsive-embed

Conflicts:
	dist/css/bootstrap.css.map
	dist/css/bootstrap.min.css
add .sr-only-focusable; fixes #12259
update docs to include+use .sr-only-focusable
Documents #10140
Merge branch 'master' into document_popover_focus
add posibillity to hover links and not background in list-groups
Fix UAs required message position on grouped radio buttons

Instead of not rendering the element at all with display:none, use opacity:0 and z-index:-1 so the radio element has a defined position on the document and user agents can show the required message in the right place.
replace .visible-{size} with .visible-{size}-{display} ; fixes #8869
Remove ./ from bower.json paths
Fixes #12683: Remove the overflow: hidden; from the .panel-group > .panel because it apparently serves no purpose and cuts off nested dropdown menus.
Merge branch 'master' into boulox-responsive-embed

Conflicts:
	dist/css/bootstrap.css.map
	dist/css/bootstrap.min.css
	docs/assets/js/customize.min.js
	docs/assets/js/docs.min.js
	docs/css.html
	docs/dist/css/bootstrap.css.map
	docs/dist/css/bootstrap.min.css
Specify engines in package.json.

Things don't work with older versions.
Remove duplicate dependency.
Add time-grunt plugin.
Moving the embed to it's own CSS file, moving the docs to the Components page with an example, fixing a Glyhpicons list problem in the docs
Merge branch 'master' of github.com:twbs/bootstrap
add responsive embed to customizer
Merge pull request #12794 from epidemian/required-radio-button-groups

Fix UAs required message position on grouped radio buttons
Comment for #12794