Migrate over to create-react-app framework and modernize deps

[toddmedema]

As part of the wide-ranging, all-encompassing upgrade, improvements include:

• Changed root div from #react-app to #root
• Removed cheerio
• Updated to material UI v5 https://mui.com/material-ui/migration/migration-v4/t
• Removed react-with-firebase-auth (deprecated) and directly coded firebase auth integration, surprisingly easy
• Assets: change image references in index.html to use %PUBLIC_URL%
• Migrated reducers and actions to toolkit's much-streamlined slicer format