Collapsible fieldsets

Resolves #46 (closed) & #47 (closed). Also introduces a new known bug (https://github.com/marmelab/react-admin/issues/4724).

Also, React-DND is a bit too low-level for this usecase and introduces a lot of spaghetti, so It'd be good to estimate similar (using Beautiful-react-dnd) solution like: https://codesandbox.io/s/final-form-arrays-handlers-error-86wem.