[A11Y] Add focus ring mixin to restore ring to elements which no longer have it (#2814)

* Add focus ring mixin

These mixins allow us to restore default browser focus rings on elements which no longer have them.

* Add info about custom outline styles; use `#private` namespace and fix mixin name

I just learned that Less has namespaces! https://lesscss.org/features/#mixins-feature-namespaces

less/common/mixins.less

@@ -1,3 +1,4 @@
+@import "mixins/accessibility.less";
 @import "mixins/border-radius.less";
 @import "mixins/clearfix.less";
 @import "mixins/light-contents.less";