Added .list-group-item.disabled

Added support for disabled List group item.
Update variables.less
Removed margin-bottom from last p of .blog-footer
.transition-timing-function Mixin
added .transition-timing-function(@timing-function) mixin, defaults to @timing-function.
Merge pull request #12924 from ahmadnassri/transition-timing-function-mixin

.transition-timing-function() Mixin
grunt
Fixes #12824: Remove white-space: nowrap from code elements
Fixes #12934
Merge pull request #12894 from lipis/patch-2

Removed margin-bottom from last p of .blog-footer
Fixes #12722: Fixes up responsive tables in print

Safari renders this fine for one reason or another, but Chrome still renders the media query styles to make tables responsive. This change scopes them to screen devices only, so printing looks boss everywhere.
Merge branch 'master' of github.com:twbs/bootstrap
Merge branch 'master' into pr/12490

Conflicts:
	less/list-group.less
Rewrite the disabled list group items to simplify styles
Merge branch 'master' into popovers-use-radii-var

Conflicts:
	dist/css/bootstrap.css.map
	docs/assets/js/raw-files.min.js
	docs/dist/css/bootstrap.css.map