- Replaces manual use of .bd-callout with {% callout [type] %}
- Rearranged some callouts for proximity to others
- Turned long lists of callouts--like those on tooltips, plugings, etc--into a list because holy shit that's overwhelming

Print with all the colors of the wind

Remove half-assed inclusion of link URLs (hrefs) when printing

* Relative URLs were output as-is, which is suboptimal or even confusing for end-users
* Ditto for fragment identifiers
* Outputting long URLs inline with prose isn't good UX

JavaScript could potentially help with this (e.g. http://alistapart.com/article/improvingprint).
However, we're not currently interested in trying to tackle the hard problem of fancy print stylesheets/views
(particularly given the level of cross-browser inconsistency when it comes to printing).
So let's just keep things simple, vanilla, and unsurprising,
which should also make it easy for others to add their own print fanciness on top of Bootstrap.

Forcing black-and-white text & backgrounds is too harsh. Printing speed isn't everything.
Users or authors can easily enable grayscale themselves if need be.

Fixes #13583: Move .sr-only and .sr-only-focusable to mixins, then use them to create classes in _utilities.scss

Conflicts:
	.gitignore
	Gruntfile.js
	_config.yml
	dist/css/bootstrap-theme.css
	dist/css/bootstrap-theme.min.css
	dist/css/bootstrap.css
	dist/css/bootstrap.css.map
	dist/css/bootstrap.min.css
	dist/js/bootstrap.js
	dist/js/bootstrap.min.js
	docs/_includes/components/navbar.html
	docs/_includes/components/progress-bars.html
	docs/_includes/css/grid.html
	docs/_includes/css/overview.html
	docs/_includes/customizer-variables.html
	docs/_includes/getting-started/accessibility.html
	docs/_includes/getting-started/browser-device-support.html
	docs/_includes/getting-started/community.html
	docs/_includes/getting-started/disabling-responsiveness.html
	docs/_includes/getting-started/download.html
	docs/_includes/getting-started/examples.html
	docs/_includes/getting-started/license.html
	docs/_includes/getting-started/third-party-support.html
	docs/_includes/js/alerts.html
	docs/_includes/js/buttons.html
	docs/_includes/js/carousel.html
	docs/_includes/js/collapse.html
	docs/_include...