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

This reverts commit d29f851e.

Making <img>s responsive by-default without opt-in can severely break
third-party widgets such as Google Maps.
This was an acknowledged problem in Bootstrap v2
(see https://github.com/twbs/bootstrap/issues/1506 ) and was fixed in v3
by requiring the .img-responsive class for explicit opt-in
(see 09cdee2f).
The situation hasn't really changed since then. The responsive-by-default
approach hasn't become any more suitable in the intervening time.

So let's avoid having this regress in v4. :-)

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.

Fixes #12424: Account for Normalize change on figure element so that we don't potentially screw folks over

Refs #12351

BS doesn't support IE6/IE7, so comma can be inserted.

Make negative margins on rows the default, not just in containers, and add padding to body to compensate

fixes #9445: make img responsive mixin use display block, but provide option to pass custom value to it

This PR significantly improves Bootstrap's accessibility for users of assistive technology, such as screen readers. Some of the these changes add additional markup to the source examples, but we believe that the sacrifice in readability is worth achieving more widespread usage of accessibility best-practices.

What was done
- Added lots of [WAI-ARIA attributes](http://www.w3.org/WAI/intro/aria)
- Added `.sr-only` helper class, that is only readable by screen readers (and invisible for all other users). This lets us - make progress bars and paginations accessible to screen reading users.
- Advised users to always use label elements. For inline forms, they can hide them with `.sr-only`
- Added 'Skip navigation' link
- Added "Accessibility" section to getting-started.html.

What *wasn't* done
- Contrast issues (twbs#3572)
- Tooltips (twbs#8469)
- Documentation re: usage of icons, since they now live in a separate repo

Major props to all that contributed: @bensheldon, @jasonlally, @criscristina, and @louh. Feel free to chime in, guys, if I've left anything out.