Fix left border on stacked cols with dividers

`.content-l_col__before-divider`s need a reduced left border width when they are converted from vertical to stacked (unless they have large gutters). I'm using a default border width on `.grid_column__left-divider` & then an override for large gutter cols in the `.stack-col` mixin to do this here (the override could also be accomplished with an `!important`  `.content-l__large-gutters` border declaration, but that might cause other issues.)