Replace the internals of `gatsby-theme-digital-garden` by using `gatsby-theme-notes` as a parent theme

[adrw]

@johno are we deprecating this repo in favor of gatsby-theme-notes?

I plan on keeping this as a standalone set of themes to continue adding new features and functionality which we can move back into gatsby-theme-blog and gatsby-theme-notes when relevant for the larger ecosystem.

I would like to replace the internals of gatsby-theme-digital-garden by using gatsby-theme-notes as a parent theme.

Originally posted by @johno in https://github.com/johno/digital-garden/pull/56#issuecomment-509783364