Move build steps for Github actions to Ruby

[numberisnan]

Setting up Github Actions to run locally is a massive pain, and maintaining effectively 2 copies of the build process (one in runlocal.sh and the other in .github/publish.yml) can lead to subtle differences in how the dev copy and production copy is built, making fixing said differences much harder.

One way to mitigate this is to have all of these things happen in the jekyll build process via plugins, and move all meaningful build steps (i.e steps needed in both local and prod) to there. This should simplify the dev run script immensely as well.