CRA 3.0 depracates babel/polyfill, react-app-polyfill is the new recommended pollyfill package.
Remove IE from the list of supported browsers and drop polyfills for it since doesn't work anyway - Proxy support in ES is required for mobx 5.x and that cannot be polyfilled.
* switch to new Sentry client lib
* Add ErrorBoundary as a wrapper around App to capture errors and display exception page with auto refresh
* rename all instances of Raven to Sentry