Build tools: Upgrade webpack to version 4.

* Minification is done by uglify, so disable that in the media build.
* The webpack boilerplate has changed, which explains the changes in the build files.
* ModuleConcatenationPlugin is enable by default for production builds so we don't have to specify that ourselves.

Merge notes: In `trunk` uglify isn't run on the media files after webpack, so webpack does need to do that. Newer webpack versions use `terser-webpack-plugin` as the default minification. Use the `uglifyjs-webpack-plugin` plugin to maintain the same behavior as before. We can look into terser as a minifier later.

Merges [43688] to trunk.
See #45065.

Built from https://develop.svn.wordpress.org/trunk@44111


git-svn-id: https://core.svn.wordpress.org/trunk@43941 1a063a9b-81f0-0310-95a4-ce76da25c4cd