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

Opening this issue to track that upstream dependency and wait for the PR at [/facebook/create-react-app #5369](https://github.com/facebook/create-react-app/pull/5369) to be merged.

Currently we're using [react-app-rewired](https://github.com/timarney/react-app-rewired) and [react-app-rewire-workbox](https://github.com/davejm/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.

Start by checking facebook/create-react-app PR #5369 and the current react-app-rewired and react-app-rewire-workbox dependencies. The work is complete when the upstream change is available and this project no longer needs react-app-rewired for service worker configuration.

Written by the indexing model from the issue text.