- 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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Used zopfli 1.0.1 with `zopflipng.exe -m --lossy_transparent -y --iterations=1000`.

Before: 199 KB (203.922 bytes)
After:  197 KB (202.151 bytes)

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* 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

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

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Replace the leftover instances with .m-x-auto (and .d-block when necessary)
Closes #19665
Refs #19102
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Fixes #17584
Closes #17607 by merging it