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

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.
fixes #10073: add width: 100%; for ie10 and below to size svg.img-responsive correctly
grunt
fixes #13020: proper color for small/.small in list group headings
fixes #13093: reset the background color and remove background image on disabled theme buttons
fix part of #13111: hide the docs nav on small and below devices
Merge pull request #13851 from ahoereth/patch-1

Add white-space: nowrap; to .dropdown-header
Merge pull request #13857 from twbs/list_group_small

List group headings and small/.small