This is semantically correct and works just as well.

* Remove IE compatibility mode meta tag from docs, examples, and JS tests as we no longer support IE9 and IE8
* update and remove some IE bits from our supported browser page
* update introduction.md to match
* reword starter template intro

* remove the $enable-flex variable option

* remove bootstrap-flex.css dist file and it's grunt task

* remove the separate flex css file for docs; it's all the same now

* remove flexbox docs (porting some to the main grid docs in next commit)

* clean up few grid docs bits to simplify copy, start to mention flexbox

* port relevant flexbox-grid.md content to grid.md

- clean up mixins
- update how it works section
- bring over sizing and alignment sections

* remove the $enable-flex from the options.md page

* update lead paragraph to mention flexbox

* update migration to mention loss of ie9 support

* remove mention of flexbox dist file

* clarify IE support

* making a note

* remove flexbox variant mentions from component docs

- updates docs for media object, navs, list group, and cards to consolidate docs
- no more need to callout flexbox variants since it's now the default

* remove $e...

* Make /utilities/ redirect to /utilities/borders/.

* Rewrite sidebar active link logic to ensure superstrings are not selected.

* Clean up some utilities

- Align CSS properties
- In `_spacing.scss`, we had a comment saying what 'a' was for, but we removed that so this comment no longer applies

* Remove '-xs' from `.navbar-toggleable-xs` and remove duplication

* Fix outdated classes to use newer ones

This is the official Google Analytics snippet.

* descriptions for getting started pages

* descriptions for layout

* add content page descriptions

* more descriptions, updates to some existing ones

* correct site url

* add social stuff to config for twitter cards

* add twitter meta tags; use large image for homepage and regular card for all others

* add the assets

* more site config

* more social shiz to partial, remove existing meta for the partial, remove page title from homepage for simpler if statements

- 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)

remove current HTML-based symbol, add any missing aria-* attributes

[skip sauce]

Ports #18872 to v4
[skip sauce]

- 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

[ci skip]

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

[skip sauce]

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

[skip sauce]

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