Use iOS 9.2 in Sauce tests

Use OS X El Capitan for Sauce tests

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Use Windows 10 for Win Chrome+Firefox tests on Sauce Labs

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Update grunt-html

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Also remove the workaround for the old validator version.

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Fixes #18493
Closes #18836
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Add prefix to footer.html search.json
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Add documentation to components/utilities to publicly expose the
.pos-f-t utility class to users.

Closes #18799
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It's very handy to add another spacer size (e.g. `4`) which can easily be done by extending the map.
Example:
```
$spacers: (
  0: (
    x:   0,
    y:   0
  ),
  1: (
    x:   $spacer-x,
    y:   $spacer-y
  ),
  2: (
    x:   ($spacer-x * 1.5),
    y:   ($spacer-y * 1.5)
  ),
  3: (
    x:   ($spacer-x * 3),
    y:   ($spacer-y * 3)
  ),
  4: (
    x:   ($spacer-x * 5),
    y:   ($spacer-y * 5)
  )
);
```

Closes #18706
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Ports #18872 to v4
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Fixes #17584
Closes #17607 by merging it

Extract $carousel-indicators-width variable

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Extract $dropdown-padding-y variable

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Extract $list-group-item-heading-margin-bottom variable

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Extract $carousel-caption-width variable

Extract $dropdown-margin-top variable

Extract $carousel-indicator{-active}-size variables

Docs: add/unify advice on contextual colors and accessibility

Replace jQuery with vanilla JS

Add $gutter parameter to make-col mixin

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Replace "label" with "tag" in navigation

- Add/replace callout for color and AT with include