Move CSS compilation to Vite Ruby

The encompasses a few changes:

1. extend the `universal_stylesheet_link_tag` and
   `universal_path_to_stylesheet` to serve styles directlt from `vite`.
1. use `vite`'s native SCSS feature. This needs a lot of config changes.
1. Use alias for @gitlab fonts: Otherwise we get files under
   `public/assets/@gitlab`. And who knows how well reverse proxies
   and such would handle built assets that have an at in the URL.