I suggest that there's no longer a need to include the section on workaround for the IE10 bug on Windows Phone 8. Not only was this bug patched in a later release of Windows Phone 8 (which shipped in 2013), but Windows Phone 8's support ended a long time ago - even its successor Windows Phone 8.1 is no longer supported by Microsoft. The potential audience affected by this bug is likely microscopic so I suggest it can be safely removed. 

I suggest the workaround JS (http://getbootstrap.com/assets/js/ie10-viewport-bug-workaround.js) can also be safely removed from the docs and examples but I haven't done that in this PR.

* Update ie10-viewport-bug-workaround.js year

* Update narrow-jumbotron copyright year to 2017

* Update carousel/index.html copyright year to 2017

* Update browsers-devices.md copyright year to 2017

* Update change-version.js copyright year to 2017

* 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 $enable-flex if/else from sass files

* remove flex dist files

* update scss lint property order to account for flex properties

* linting

* change to numberless classes for autosizing, wrap in highlighting div

* bump gruntfile and postcss to ie10

* redo intro sections

* rearrange

* phew, redo hella grid docs

- rearrange all the things
- consolidate some bits

* remove reference to flexbox mode

* more border action for demo

* Make some changes to the .card's in .card-deck's to ensure footers align to the bottom

* 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

Refs https://bugs.chromium.org/p/chromium/issues/detail?id=273306#c34

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

Refs #14839, #19088
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The modal backdrop positioning bug related to iOS' virtual keyboard doesn't seem to reproduce in iOS 8.0+.
(Possibly as a side-effect of https://bugs.webkit.org/show_bug.cgi?id=153224 )
Refs #9023
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* https://bugs.chromium.org/p/chromium/issues/detail?id=273306
* https://bugs.webkit.org/show_bug.cgi?id=138192
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https://bugs.chromium.org/p/chromium/issues/detail?id=175502
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Refs #19077
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Replace inline HTML with markdown, rewording, and cleanup.
Closes #18984

Refs #18984
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If these are supported, why bother mentioning them in a section that's about communicating what's not supported?
These rows made sense when the section also covered IE8 (http://getbootstrap.com/getting-started/#support-ie8-ie9 ),
which doesn't support these features, but we no longer care about IE8 in v4.

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Per http://caniuse.com/#feat=documenthead , document.head is supported in IE Mobile 10+,
so slightly simplify the JS accordingly.

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Fix #18076

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add flexbox to the not supported list 

https://github.com/twbs/bootstrap/issues/17802

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