Mobile first: Apply squaregrid to limited screen sizes

This entails defining screen limits for squaregrid.css, moving css loading to a configuration option. I am folding this into checkboxes to select support for various screen sizes.

No classes are changed in doing this, so I do not expect this to break any child themes. Themers will want to double-check their theme configuration settings presented by squaregrid to ensure that squaregrid is passing along expected css.