Koa upgrade

This PR makes us ready for using Koa. The build-images script includes steps for enabling Figures, provided that you use the following fork and version of the tutor-figures plugin:

https://github.com/vrook-co/tutor-figures/commit/8ab5383f4cc028df133392e18bab1df799daa076

(instructions are also added to the README)

This MR also removes a bunch of extra files that are generated by Tutor and we probably don't want to have in the repository.