Add a way to configure i18n messages (all Nuxeo version)

> add a registry for i18n
> add a key<>value table
> add a way to access this screen through a link which contains a i18n key
> if the link contains an existing key, edit the current value
> otherwise create the corresponding entry
> add a save/discard button
> generate the deployement fragment part to contribute to i18n if some keys are customized
> for each constraints, replace the given i18n key by a link to this registry