Update reverse approach

Use rtl to ptr to achieve reverse directions for columns at small
views. This approach preserves .row widths in hybrid and fixed.
Make min-height on main default behavior
SASS output
Spine CSS as a table of contents

Building Spine CSS as a table of contents from partials, so the
different parts are not obscured by multiple build steps. Additional
reorganization can happen in future PRs, so it's easier to integrate
changes at deeper levels, and so we get the advantages of smaller files
earlier.