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/components/progress-bars.html
	docs/_includes/components/responsive-embed.html
	docs/_includes/components/thumbnails.html
	docs/_includes/components/wells.html
	docs/_includes/css/buttons.html
	docs/_includes/css/code.html
	docs/_includes/css/forms.html
	docs/_includes/css/grid.html
	docs/_includes/css/helpers.html
	docs/_includes/css/images.html
	docs/_includes/css/less.html
	docs/_includes/css/tables.html
	docs/_includes/css/type.html
	docs/_includes/customizer-variables.html
	docs/_includes/getting-started/accessibility.html
	docs/_includes/getting-started/disabling-responsiveness.html
	docs/_includes/getting-started/download.html
	docs/_includes/getting-started/whats-included.html
	docs/_includes/js/alerts.html
	docs/_includes/js/buttons.html
	docs/_includes/js/carousel.html
	docs/_includes/js/collapse.html
	docs/_includes/js/modal.html
	docs/_includes/js/overview.html
	docs/_includes/js/popovers.html
	docs/_includes/js/tabs.html
	docs/_includes/js/tooltips.html
	docs/_includes/nav/components.html
	docs/_includes/nav/javascript.html
	docs/_jade/customizer-variables.jade
	docs/_layouts/default.html
	docs/about.html
	docs/assets/css/docs.min.css
	docs/assets/css/src/docs.css
	docs/assets/js/customize.min.js
	docs/assets/js/raw-files.min.js
	docs/assets/js/src/customizer.js
	docs/customize.html
	docs/dist/css/bootstrap-theme.css.map
	docs/dist/css/bootstrap.css
	docs/dist/css/bootstrap.css.map
	docs/dist/css/bootstrap.min.css
	less/glyphicons.less
	less/mixins/vendor-prefixes.less
	less/navbar.less
	less/popovers.less
	less/tables.less
	less/theme.less
	less/tooltip.less
	less/variables.less
	package.json
	scss/_carousel.scss
	scss/_close.scss
	scss/_forms.scss
	test-infra/npm-shrinkwrap.json

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.

Remove extra word from disabling-responsiveness.html

Closes #14937
[skip sauce]

Closes #15288

[skip sauce]