- 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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Ports #18872 to v4
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- Add/replace callout for color and AT with include

Capture the slug once before applying it and add a slash to it to so we get a unique string to match against instead of a fuzzy partial 'contains'. Helps avoid 'grid' highlighting 'flexbox-grid', for example.

content had already been wrapped in the `<div class="col-md-3 col-md-push-9 bd-sidebar">`

Use the structure as documented in
http://v4-alpha.getbootstrap.com/components/navbar/#collapsible-content
- wrapping both the toggle and the navigation in a `<nav>`, using a
`<ul>` for the navigation items

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For easier debugging.
Just like we already do for the JS.

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We can't include it in docs.min.js because docs.min.js includes application.js,
application.js depends on bootstrap.js,
and the tooltip portion of bootstrap.js depends on Tether.

So instead, we need to load Tether separately before bootstrap.js

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Now that we don't support IE 8 it's safe to do that.

Fixes potential problems with iOS9's new viewport behavior - see for
instance https://forums.developer.apple.com/thread/13510

Move them to footer, and remove duplicate ones.

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Related to #17314.

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1) Update it to v1.0.8
2) Include jekyll-search where it's used only

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- Simpler main nav on all pages
- Back to purple masthead on homepage instead of dark graphite
- Active link styles on the main nav
- Cleaned up sidebar nav
- New docs layout name
- Homepage copy edits
- Updated bright purple docs color