Add the possibility to keep the js class names in the production build

Created by: svenliebig

I added an environment variable to prevent the terser plugin from minifying js class names in the production build.

The variable is: KEEP_CLASS_NAMES - i added an example in the documentation, beneath the production-build documentation.

There is no major change, the production build will be equal to the production build before this pull request, if there is no environment variable set.