Preserve the link and image references when saving the content

When saving the content the link and image references are replaced with URLs. Cristal's approach is to convert those URLs back to entity references. We can provide an implementation for this, but it would mean that all references are going to be normalized on saved. On the long run we need to find a way to pass the link and image references in the edited content and use them on save, in order to preserve exactly the form from the source, unless they were modified explicitly by the user.