Enable hot reloading - Fixes #2134

[lindapaiste]

Fixes #2134

Changes:

• Replace deprecated package react-hot-loader with react-refresh.
• Integrate react-refresh with webpack using package @pmmmwh/react-refresh-webpack-plugin.
• Follow this example setup.
• Replace webpack-hot-middleware with forked version @gatsbyjs/webpack-hot-middleware, which is up to date for webpack v5.
• Minor version bump of react and react-dom to the highest v16 version (to fit these recommendations).

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

This PR improves the developer experience by allowing changes in the code to show up immediately without reloading the whole page. React and redux state will stay active even when code is changed.

The only weird thing is that hot reloading messages from the embedded preview app will show up in the editor console. I don't think it's a big deal.

[raclim]

Thanks so much for your work and sorry it took so long to get to this, this really does improve the developer experience! This looks great to me so far besides the portion that I commented on!

[raclim]

Looks great to me! I might be wrong, but I think the hot reloading messages disappear once some of the deprecation warnings are addressed.