Upgrade webpack to webpack 4

[epicfaace]

This will also fix the vulnerability in webpack-dev-server (it's only local, won’t affect production).

Upgrading webpack-dev-server to 3.11.1 will also require upgrading to webpack 4, which will require a lot of changes in configuration.

Use the same webpack configuration that is already used in https://github.com/TreeHacks/application-portal-frontend rather than using the ejected create-react-app scripts in the scripts directory.