Convert to flake

This also changes how reveal.js is vendored and as a consequence of this
change, it needs to dereference the build result before deploying it
with netlify. This is because flakes do not like submodules and netlify
does not like symlinks.

.drone.yml

@@ -17,6 +17,6 @@ steps:
       NETLIFY_AUTH_TOKEN:
         from_secret: netlify_auth_token
     commands:
-      - cp -rL result netlify-does-not-follow-symlinks
+      - cp --no-preserve=mode -rL result netlify-does-not-follow-symlinks
       - nix develop -c netlify deploy --prod -d netlify-does-not-follow-symlinks
       - rm -rf netlify-does-not-follow-symlinks