Add a variable for .dl-horizontal's offset

List group headings and small/.small

Add white-space: nowrap; to .dropdown-header

Darken @gray-light to meet accessibility guidelines

Missed this in 4cbc8d49

`@gray-light` darkens from #999 to #777, which puts `.text-muted` at the
threshold for the 4.5:1 WCAG minimum contrast[1]. #777:#fff is 4.48:1.

The “Example” headers in docs become #959595, which is contrast ratio
3:1, the minimum for larger text. Since the headers are less important
than the surrounding text, 3:1 is fine and an improvement on the
previous #bbb:#fff (1.92:1).

Fixes issue #13847.

[1] http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html

Dropdowns adjust their width to the width of their containing a elements - which have no line breaks ([*](https://github.com/twbs/bootstrap/commit/f226463b057f945795661c3d19fbcabf63b931f7)). Consistent to this dropdown-headers also should be prevent to contain line breaks.

update "is jQuery defined" check for UMD compatibility

Also, remove deprecated --use-mirrors option

Refs #13842

Closes #13782 by merging it.

Fix dismissible spelling

Closes #13795 by merging it.

Refactoring bages in README.md.

bump load-grunt-tasks to ~0.5.0

[skip validator] [skip sauce]

Also, makes little sense to say that Firefox sets a `-webkit`-prefixed value