Update fether-ui to use react-scripts instead of solely on Babel to minimise usage of Babel

[ltfschoen]

Quote from https://github.com/paritytech/fether/pull/332#pullrequestreview-199990668

The idea of fether is to have (almost) 0 configuration, i.e. no babel at all. So ideally:

• fether-electron uses electron-webpack
• fether-react uses react-scripts
• fether-ui should also use react-scripts (TODO)

Right now:

• fether-react needs decorators, so we manually add craco and babel inside fether-react to make that work.
• fether-ui relies solely on babel (we should switch to react-scripts).

The bottom line is: we should aim for the least amount of babel config inside this project.

[amaury1093]

The idea is:

• We don't do any compilation inside fether-ui
• Since react-scripts@2, we can import 'some-module', with 'some-module' written in es6. That's what we are using
• Change this line to src/index.js