Modernize ofborg-viewer

[zebreus]

This PR modernizes the stack used by ofborg-viewer and automates deployments. It also adds darkmode. You can view a deployed version with this PR at ofborg.wirs.ing (deployed via GH actions to GH pages from my fork).

Changes:

• Use flakes with a recent version of nixpkgs
• Use nodejs 20 instead of nodejs 6
• Use buildNpmPackage instead of yarn2nix
• Switch from webpack to vite (because updating from webpack 2 was too much work)
• Update eslint & add prettier
• Simplify deployment with github actions and github pages
• Uses normal css instead of less
• Use darkmode if the user prefers-color-scheme: dark

Deployment should be as easy as merging this PR, setting GH pages to deploy from actions, and pointing logs.ofborg.org to GH pages.