[autoform] Make RelationField.fromcardinality a class method factory Instead of having the "RelationField" class hard-coded in this factory method (thus preventing any subclass to properly use it), we make that method a classmethod and instantiate the actual class instead of the base one.

cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)