refactored and renamed grid mixins
grunt with new mixins
grunt
Merge branch 'master' into pr/12412

Conflicts:
	dist/css/bootstrap-theme.css.map
	dist/css/bootstrap.css.map
	dist/css/bootstrap.min.css
	docs/assets/js/customize.min.js
	docs/assets/js/raw-files.min.js
	docs/dist/css/bootstrap-theme.css.map
	docs/dist/css/bootstrap.css.map
	docs/dist/css/bootstrap.min.css
	test-infra/npm-shrinkwrap.canonical.json