Drop the $small-font-size variable for the newer $font-size-sm one

Update the grid table heading to clarify max width over width

v4: The Utilities Update

v4: Grid adjustments

Creating max-width images is not dependent on the display, so setting it is redundant. Cleans up the comments and implementation of the mixin as well.

Fixes #20767

Fixes #18510

Makes it so that the container is no longer wider than the breakpoint used in the media query. This was never really an issue in rendering as the max-width handled it appropriately, but the mismatch was still incorrect.

Fixes #18054

- Fixes issues between float and flex grid systems where container wouldn't fill the available width in Chrome & FF (but would in Safari)
- Fixes #20681
- Fixes #17621 (basically same issue as above issue)

Originally -font-size was added in v3 to set the font-size of the <small> element. Now that we have newer, global type variables, we can use those instead for these kind of things.

Fixes #20859

Fixes #20688

[ci skip]

[ci skip]

See https://bugs.webkit.org/show_bug.cgi?id=158342
Fixes #17695

With apologies, copy/paste error following on from trying to fix conflicting Hound/Travis checks for https://github.com/twbs/bootstrap/pull/20821/ slipped through before I noticed them.

Update print styles borrowed from h5bp

Change `hidden-sm-up` to `hidden-md-up` as the clearfix divs should be visible on both `xs` and `sm` devices (widths of 6 columns).

* Fixes #20775 without adding extra width and an important flag

* Since we're now getting the extend, we don't need the extra position relative

* rerrange

* getting min-height from the extend already

Merge branch 'customizable-grid-gutters-per-breakpoint' of https://github.com/pixelbandito/bootstrap into pixelbandito-customizable-grid-gutters-per-breakpoint