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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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