gatsby-remark-images causes slow build and OOM

[vnugent]

The build is very slow during Source and Transform stage following the introduction of gatsby-remark-images

https://github.com/OpenBeta/open-tacos/blob/develop/gatsby-config.js#L66

Out of memory when doing a full build (./prebuild full).

Todos:

• [x] Try gatsby-transformer-remark instead of mdx plugin
• [ ] Prototype a nextjs site (drop Gatsby)

[vnugent]

Build on Gitlab runner now takes 8.5 mins.

[vnugent]

Will create a separate issue for Next.js prototype.