Include webpack-subresource-integrity plugin

https://www.npmjs.com/package/webpack-subresource-integrity

This plugin has two effects, one for HtmlWebpackPlugin which doesn't affect but also provides support for chunks created when we're using code splitting. It adds `integrity` and `crossorigin` attributes to dynamically loaded scripts with the hash of the loaded file.

It's an added security feature and will be important to use in conjunction with any future sites that want to use the `require-sri` content security policy.

Notes:

- You have to set the `output.crossOriginLoading` parameter, use "anonymous" as the value
- Only need to use a single hash function eg. `hashFuncNames: [ 'sha384' ]` is a good default