Created by: cvrebert
Adds .visible-{screen size}-block, .visible-{screen size}-inline, and .visible-{screen size}-inline-block classes (and also a corresponding trio for .visible-print). Also deprecates the .visible-{screen size} classes.
Proposed fix for #8869 (closed) pursuant to #9397 (closed). Includes docs updates.
There is most likely a way to refactor this to be less repetitive & more concise by using some more sophisticated Less features, but this should be okay for the time being.
/to @mdo for review.