[#71053354] proof-of-concept SHL styling

- added disabled import of shl.less
  - and excluded out-of-tree file from git
- changed @brand-primary colour, tweaked .btn-primary's color:
Merging in upstream changes after Git mess
Merge branch 'master' of github.com:twbs/bootstrap

Conflicts:
	dist/css/bootstrap.min.css
	docs/assets/js/customize.min.js
	docs/assets/js/raw-files.min.js
	docs/dist/css/bootstrap.min.css
Fixed git mess again