Update gruntfile to compile the Sass extras (flex, grid, and reboot) as part of the dist-css task (#20682)

- 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)

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Only grunt-saucelabs is left in order to update grunt to v1.x.

* bower.json, package.json: Extend jQuery version ranges to include v3
* NuGet: Bump jQuery to v3.0.0.1
* Docs+Examples: Update jQuery to v3.0.0
* Use jQuery v3.0.0 for JS unit tests
* Update jqueryVersionCheck to allow jQuery v3.x.x

Previously, when running the docs locally, the site, rooted at:
    http://localhost:9001/
would reference docs assets using relative URLs such as:
    /../assets/js/vendor/anchor.min.js
which is equivalent to:
    http://localhost:9001/../assets/js/vendor/anchor.min.js
which is nonsense, since the root directory has no parent directory.
Apparently browsers silently ignore this extra '..', hence why this wasn't noticed until now.
But if you adjust Jekyll's `baseurl` setting, this mistake causes incorrect URLs to get generated.

This commit corrects the problem by removing the extra '../' from the paths.

These paths are also referenced in the Gruntfile, where the fix actually allows us to simplify the code.
Previously, in the Gruntfile, we were doing, e.g.:
    path.join('./docs/assets', '../assets/js/vendor/anchor.min.js')
which calculates to:
    ./docs/assets/../assets/js/vendor/anchor.min.js
which can be simplified to:
  ...

Refs #19990
Continues the degruntification process.

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

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Refs #19990

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Refs #19908

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

Use concat's process function instead.

Also remove the workaround for the old validator version.

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Remove csscomb (soon replace it with something else) because scsslint has most of it covered already

Workaround for https://github.com/jzaefferer/grunt-html/issues/86

Put all the textual input types right next to each other for easy visual comparison.
Refs #17308, #18763

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Erratum from #18778
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Refs #18584
Closes #18659

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Ports #18445 to v4.

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The latest OS X Safari version is 9.0.1
Our prefixing policy dictates that we prefix 1 version back from the latest version.
9.0.1 - 1_major_version = 8.0

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Should automate the release process.

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The latest iOS version is 9.1
Our prefixing policy dictates that we prefix 1 version back from the latest version.
9.1 - 1_major_version = 8.0

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Adapt uglify for the new version.