Add Sass variable for prefers-reduced-motion, add callout to affected components (#27581)

* Tweak the accessibility/reduced motion text

include mention of carousel slides, remove the (now inaccurate, as Firefox 63 includes it too) mention that support is limited to Safari/macOS

xref https://github.com/twbs/bootstrap/issues/27525

* Add new callout for reduced motion

* Add variable to control prefers-reduced-motion media query support

* Add callout about prefers-reduced-motion to all components currently using animation which are affected

site/docs/4.1/components/modal.md

@@ -23,6 +23,8 @@ $('#myModal').on('shown.bs.modal', function () {
 })
 {% endhighlight %}
 
+{% include callout-info-prefersreducedmotion.md %}
+
 Keep reading for demos and usage guidelines.
 
 ## Examples