remove deno and init nix

[jackjohn7]

• remove deno (rustyscript) dependencies
• build only a single docker image
• begin setting up a nix environment

Nix

The goal with this is to simplify our build process and our CI/CD in the near term while transitioning to a cheaper, better-cached nix-driven solution for CI in the medium-long term future.

Nix isn't becoming a requirement for building the application. The app will always be able to be built from source on generic Linux (landlock necessitates Linux). Nix will just be a measure to ensure our CI becomes cheaper (by super efficient caching) and our builds become more easily reproducible.

Deno

We thought a JavaScript runtime would be a fairly simple way to add some neat functionality, but we've come to realize that such a capability is simply not necessary. While it could enable one to do some neat things, I think webhooks end up being a better abstraction to enable third-party integration. We'll need to support more API functionality for consumers though.