Fix issue #15601 — Position of X-UA-Compatible, charset and viewport

Add comment to all examples

The non-responsive is correctly saying 2 instead of 3

Use a simple blank.gif and change the background color instead of duplicating stuff.

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plus one example that still used the old <div role="navigation"> and a
fix-up of the callout as per
https://github.com/twbs/bootstrap/issues/15109

Fixed a typo in Sign-in page example
http://getbootstrap.com/examples/signin/

adding explicit aria-expanded="false" initial state (so they're
correctly announced by AT as being collapsed), and - in the case of
old-school use of <a> - role="button"

plus minor tweak to customizer's generated alerts

Closes #14951 by merging it.

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Added handling of aria-expanded=true/false to collapse.js, updated
documentation to include advice on making expand/collapse controls
accessible, updated examples and javascript documentation to use
aria-expanded and aria-controls (when targetting single collapsible
element, using ID rather than class selector)

Closes #14147.
Closes #14153.

Fixes #14500

Refs #13554.
Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#tab-panel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin

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Refs #13556.
Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#carousel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin

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Refs #13556.
Per https://github.com/paypal/bootstrap-accessibility-plugin/#carousel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin

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Follow-up to e9374c06
Per https://github.com/paypal/bootstrap-accessibility-plugin/commit/f203dea4b6d17264d7b776f02feaeeaa386cc9b2
Credit: https://github.com/paypal/bootstrap-accessibility-plugin & @rohk

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Moved the inset box-shadow to .site-wrapper in order to keep the shadow at the edges even if content exceeds viewport and needs scrolling.

This way, it works in all browsers.