Grid classes have been changed to account for a fourth set of classes,
meaning we now have XS, S, M, and L options. Specifically, we made the
following changes:

* Renames `.col-` to `.col-xs-`
* Adds `.col-md-` for devices 992px wide and up
* Remaps `.col-lg-` for devices 1200px wide and up

Alongside that, we've updated our media queries to better handle
advanced grid layouts:

* All column classes now kick in when they hit a `min-width`, undoing
the #9128 fix in
https://github.com/twbs/bootstrap/commit/db45a60cc86190a060f0baf0b3961cb
c690f3a65
* All column offsets, pushes, and pulls are scoped to an appropriate
range, meaning a `min-width` and `max-width` (thanks for the idea,
@eratzlaff)

We've also modified the widths of our grid containers for the small
device grid (now a max 720px instead of 728px) and large device grid
(now a max 1140px instead of 1170px) to avoid horizontal scrollbar
problems in Firefox (likely due to box-sizing, which is why we didn't
see it in 2.x).

Similarly, we've updated the responsive ultility classes to match the
new four media query approach of the grid system. That means we've:

* Added new `.visible-xs` and `.hidden-xs` classes
* Reassigns visible and hidden classes for small, medium, and large to
matching grid system media queries
* Updates docs examples to match grid system and account for fourth
utility range

-----

Fixes the following:

* #9135: fourth grid tier
* #9302: undoes previous range change for small grid system, so no need
to update any docs here
* #8755: consistent grid and responsive utilities schemas)
* #9195, #9216, and #9227: no more horizontal scrollbars

http://blog.paciellogroup.com/2013/02/using-wai-aria-landmarks-2013/#tablex

http://www.w3.org/TR/wai-aria/roles#complementary

http://blog.paciellogroup.com/2013/02/using-wai-aria-landmarks-2013/#tablex

http://www.w3.org/TR/wai-aria/roles#banner

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.

I reworked the customizer to match the new docs layout and include all
newly added variables, etc.
Would be glad to keep an eye on Customize and update it as v3 changes.
Help blocks in my new version need some more work, too, so this is just
a basis.