Move the `onChange` handler back to `Radio` inputs

[adamkudrna]

Currently, the onChange event handler is tied onto the wrapping <fieldset> which causes React to throw the following warning:

Warning: You provided a `checked` prop to a form field without an `onChange` handler.
This will render a read-only field. If the field should be mutable use `defaultChecked`.
Otherwise, set either `onChange` or `readOnly`.

The onChange handler needs to be transferred onto individual radio inputs as it was before #466.

Don't forget to make the docs more obvious: https://react-ui.io/components/radio#forwarding-html-attributes