Blank .hidden-print class doesn't have sense.

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

This change prevents situations where specificity causes some rules to
be overridden by the ones intended for mobile.
* Added mixin `responsive-invisibility()`
* Swapped out `display: none !important;` for new mixin

* Update docs language
* Enforce display: block; on all utilities for now
* Fix up docs CSS for the tests

Fixes #7577: Attempt to fix, at least, issues with Windows Phone 8 and responsive not kicking in correctly

1. Rename file from responsive-utility-classes.less to responsive-utilities.less
2. Add additional help text around the documentation for the classes in Scaffolding
3. Remove unnecessary display values on initial classes
4. Drop block values for inherit for better support on inline and table elements
5. Make use of \!important on every class to avoid classes with more specific selectors (e.g., .span* classes for the grid)