Add cssbundling-rails and remove sassc-rails from the GitLab project.

`cssbundling-rails` enhances `assets:precompile` to invoke `yarn
build:css`. Ran the setup command for `cssbundling-rails

Added a rudimentary `build-css` script to compile assets using Dart Sass
NodeJS

because `sassc-rails` is not being used, we have to load
`sprockets/railtie` ourselves

Changelog: changed