Refactor the JSONEditor component

[n1k0]

Spwaned from #204:

For JSON fields, it would be nice if the stringification and parsing would be done in the same place (both at the form level) : https://github.com/Kinto/kinto-admin/blob/v1.3.0/scripts/components/BucketForm.js#L79-L80 https://github.com/Kinto/kinto-admin/blob/v1.3.0/scripts/components/BucketEditForm.js#L31-L32

[alexcottner]

Closed with PR 3043.