Use Webpack's watch mode or module replacement functionality

[podliashanyk]

Since #2849 CSS files are built from Sass using Webpack.
sass-watch.sh should be replaced with Webpack's functionality for module replacement or watch mode. We can most likely remove the sass-watch script entirely, but the inotifywait will still be needed in Docker setup as it is used for module replacement by Webpack under the hood.

Blocked by #2849