Upgrade to Gatsby 3.x.x

[merit-mthompson]

Our version of Gatsby (2.25.4) is now old and we should look at moving to 3.x.x. As of right now most things should be fine but there are plugins we use that haven't been updated. Once these are updated, we should upgrade also.

We haven't gone through every plugin, but right now it's "gatsby-plugin-advanced-sitemap" that's the first up as a blocker and is being worked on in TryGhost/gatsby-plugin-advanced-sitemap#146

[merit-mthompson]

There's a migration guide at https://www.gatsbyjs.com/docs/reference/release-notes/migrating-from-v2-to-v3/ but right now things will fail until the packages we use are updated.