Upgrade to Webpack 4

[johngrimes]

We're using quite an old version of Webpack, and some of our loaders and plugins are causing problems for security updates because of the old transitive dependencies they bring in to the bundle.

This task is to:

1. Update to the latest version of Webpack
2. Update all Webpack loaders and plugins
3. Make any necessary updates to the Webpack configuration
4. Refactor and simplify where appropriate