Tracking upstream Workbox CRA config and our react-app-rewired dependencies

[boomskats]

Opening this issue to track that upstream dependency and wait for the PR at /facebook/create-react-app #5369 to be merged.

Currently we're using react-app-rewired and react-app-rewire-workbox to enable customisation of our service worker config, so we can control the offline mode, update and caching behaviours without ejecting. This won't be needed once that PR is merged and we can get rid of the dependency on react-app-rewired.