Make each layout aware of different behaviors

Layout transitions between large, medium, and small at different points
depending on selected responsive behavior. “Large” is default
appearance. At “Medium”, the spine collapses and becomes a header
spine. At “Small”, most columns stack and become full width. We are
targeting these basic layouts rather than specific viewport widths. To
be able to target differences between behaviors, a pixel width
designation is also present where there is divergence between
behaviors. This should account for all scenarios.
Preserve the smallish class until officially deprecated

Let’s be very conservative and officially deprecate and announce before
removing the smallish class.
Introduce @jeremyBass refinements
Merge pull request #220 from washingtonstateuniversity/sizing-classes

Make each layout aware of different behaviors