Update gs-container

This is so that breakpoints on content pages match those on fronts on NGW.
Oh dear… good point. I wonder if the gs-container parts of this project should be removed, and implemented separately within each application separately (a breaking change). Or we could extract gs-container into a(nother) paste-up project. Or perhaps gs-container can become more of a helper class, that take breakpoint/width values, but has a set of defaults. Options! Will discuss with @sndrs on Monday.