- Restores two-mixin approach to generating semantic grid columns (now with 'make-col-ready' and 'make-col')
- Removes need for .col-xs-12 by restoring the mass list of all grid tier classes to set position, min-height, and padding
- Adds an initial 'width: 100%' to flexbox grid column prep (later overridden by the column sizing in 'flex' shorthand or 'width') to prevent flexbox columns from collapsing in lower viewports

As this causes non-standard dotted outline on checkboxes/radio buttons
in OS X Firefox

Ports #19935 to v4
Refs #19933

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Refs #19990
Continues the degruntification process.

Also removes mq4-hover-shim for now,
since it doesn't yet implement the standard PostCSS plugin interface.

- Remove background-color from header and footer in inverse cards (fixes #19841)
- Update border override for header and footer in cards; only need to declare a new color
- Update the selector for targetting blockquote footers in cards

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fixes #19148 as follow up to #19099: ensure local mixin variable is used for  in make-grid-columns mixin

Uses a new more specific class, adds some basic spacing, and clarifies
that this class is only for validation feedback.

Clears up the help text class question in
https://github.com/twbs/bootstrap/pull/18704#issuecomment-168241731 and
addresses the vertical spacing in #18703.

Alternate fix to #17965: Updates to use the new class name and adds some basic docs for custom checkbox/radio validation

- Both classes do about the same thing, but with different names
- Clarifies docs for .m-x-auto requiring a fixed width block element for it to work
- Add missing heading for clearfix section in docs (unrelated)

- Flexbox responsive behavior fixed with specific .col-{breakpoint} classes now added
- Dropped the make-col mixin in favor of a column-basics placeholder that we can extend across our grid infrastructure
- Updated docs to use required .col-xs-12 (as a safeguard for when folks enable flexbox mode--this isn't necessary in default grid mode)
- Update flexbox grid docs to include responsive docs, tweak some other bits too

flip things around again, no more base class, try out some new stuff to keep responsive flex grid working

Revamp grid classes to require .col base class and improve flexbox grid classes with auto layout via flex: 1

- Rename -height to -height-base to match other vars
- Drop use of -height across the board and rely on it to be inherited
- Adjust padding of .dropdown-header to account for different line-height of h6 heading element (this needs refactoring for variables and rems also)

Fixes #16871 for v4

gradient-{horizontal=>x}[-three-colors]
gradient-{vertical=>y}[-three-colors]

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In SCSS, the quotes were included verbatim in the resulting CSS, which isn't valid syntax for the `font` property.
Removing the quotes fixes the syntax error and does not cause any SCSS compiler error.

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cause hover isn't working since bg-color is !important

Fixes #18036

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