Add site sizing variables to theme

[sarahmonster]

I seem to keep adding this to repos (octopusthink.com, getmicdrop.com, turniptimer.com), so it might make sense for us to add to Nautilus as well:

  // Extra site config type stuff.
  site: {
    mobilePadding: '1.6rem',
    tabletPadding: '3.2rem',
    desktopPadding: '6.4rem',
    maxContentWidth: '64rem',
    maxSiteWidth: '120rem',
  },

This would be useful for lots of different components as well, so worth including as part of our native theme as we start building up larger components (Panels #248, for instance!)

[sarahmonster]

Duplicate of #194; whoops.