Add site spacing variables to theme

I keep adding these to sites I'm building using Nautilus, so we should include them and make it official-like! It's also better than using a config value in Gatsby sites, because it's simpler/more straightforward to access. (See: no GraphQL queries!)

https://github.com/octopusthink/octopusthink.com/blob/1b013d633c272d9ba8bd8854a1485e9485361350/data/theme.js#L295

  // Extra site config type stuff.
  site: {
    mobilePadding: '1.6rem',
    tabletPadding: '3.2rem',
    desktopPadding: '6.4rem',
    maxContentWidth: '72rem',
    maxSiteWidth: '120rem',
  },

We may want more, but those are the standard ones I tend to use.

octopusthink / nautilus

Add site spacing variables to theme #194