Use esbuild to minify CSS (go-gitea#15756)

It's about a 30% speedup in webpack build time with neglible differences
in the output size. We do lose the ability for CSS source maps, but I
rarely have a use for them anyways.

Co-authored-by: techknowlogick <[email protected]>
Co-authored-by: 6543 <[email protected]>

Author: 3 people