Cards must be inline-block in columns, otherwise they can get split across columns

- Update docs examples to use newer .form-check suite of classes
- Update form validation mixin to use newer classes as well

See: https://github.com/twbs/bootstrap/pull/18255#issuecomment-237034763

Instead of the regular border-radius var, use the input ones to match sizing with their associated controls.

- Creates new flexbox grid Sass file in our docs assets
- Updates the Gruntfile to compile said new Sass file and minify the output
- Update notice on flexbox docs page for how it works
- Only enable compiled flexbox grid CSS in hosted docs site, not in dev (for easier and specific debugging of all flexbox features)

- 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

[skip sauce]

SCSS-Lint 0.49.0 modifies Shorthand linter to report lint if a shorthand
of a length not specified in the allowed_shorthands option is used.

New defaults include "4" in the list of allowed shorthands, so we are changing our configuration accordingly.

Ref: https://github.com/brigade/scss-lint/commit/e283d1689699f581561fea344df3168128c46d7b

Refs #19990
Continues the degruntification process.

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

[skip sauce]
[skip validator]

️️️ https://github.com/twbs/bootstrap/graphs/contributors

Here we're doing some margin swapping, so it looks a little funky. All this does is match the margin implementation and rendering across our table and flex versions of card decks.

- 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

- Remove the box-shadow and switch back to border to match .card basics
- Update the header nav margin override--since we restored the border, we need that default negative margin at the bottom

- Add inverse examples for all variants
- Use rgba() colors for hover and accent states for easy usage in inverse styles

- Really we didn't need to do any of that strip units stuff to generate six new variables, two for each button size
- Using sibling selectors, we can target those split button dropdown toggles using the .dropdown-toggle-split class, and adjust padding and margin as needed
- Now, we nuke the margin-left from the ::after generated caret and tighten up the padding so that those split toggles don't look huge next to their main button

- Includes new class
- New docs to support it
- Rearranged older docs