Overhaul of responsive visibility classes

Thanks to @glebm for help refactoring this to use the breakpoint mixins.

replaced media-xs => media-breakpoint-down(xs)
replaced media-sm => media-breakpoint-up(sm)
replaced media-sm-max => media-breakpoint-only(sm)
replaced media-md => media-breakpoint-up(md)
replaced media-md-max => media-breakpoint-only(md)
replaced media-lg => media-breakpoint-up(lg)

Use improved box sizing reset

Based on http://css-tricks.com/inheriting-box-sizing-probably-slightly-better-best-practice/
This allows us to significantly simplify the relevant part of the docs.

Remove affix from docs & CSS; add migration guidance

bump min OS X Safari in Autoprefixer config to v7.1

Conflicts:
	Gruntfile.js
	dist/css/bootstrap-theme.css
	dist/css/bootstrap-theme.css.map
	dist/css/bootstrap-theme.min.css
	dist/css/bootstrap.css
	dist/css/bootstrap.css.map
	dist/css/bootstrap.min.css
	dist/js/bootstrap.min.js
	docs/_data/glyphicons.yml
	docs/_includes/components/alerts.html
	docs/_includes/components/badges.html
	docs/_includes/components/breadcrumbs.html
	docs/_includes/components/button-dropdowns.html
	docs/_includes/components/button-groups.html
	docs/_includes/components/dropdowns.html
	docs/_includes/components/glyphicons.html
	docs/_includes/components/input-groups.html
	docs/_includes/components/jumbotron.html
	docs/_includes/components/labels.html
	docs/_includes/components/list-group.html
	docs/_includes/components/media.html
	docs/_includes/components/navbar.html
	docs/_includes/components/navs.html
	docs/_includes/components/page-header.html
	docs/_includes/components/pagination.html
	docs/_includes/components/panels.html
	docs/_includes/compone...

Happy New Year

Document that responsive tables use `overflow-y: hidden`

Document that Autoprefixer is required

Address #15419

account for XL size tier in grid docs

Use a glyphicon instead of the unicode character.