Fixes #18044

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Refs #15947, #17542

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This reverts commit 61067304.

The normalize.css pull request (https://github.com/necolas/normalize.css/pull/379 )
was REJECTED, *NOT* merged!

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Refs #18178

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Refs #15947
Somehow c256aca1 got lost during the SCSS translation...

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They can be rounded later on if $enable-rounded: true.

Perhaps I'm a little slow & need to have things spelled out :)
https://github.com/twbs/bootstrap/issues/17301#issuecomment-134469886

Fix #17052

See https://github.com/twbs/bootstrap-sass/pull/805

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.