cubicweb-container #2510552 also clone the container subject relations if needed [resolved]

This is not done because the clone form handles these, but a more general solution must be provided (handling container subj rels in the clone form AND completing what's missing otherwise).

prioritynormal
typeenhancement
done in0.3.0
load1.000
load left0.000
closed by#c89bcc7ea19b [entities] also clone subject relations of the container (closes #2510552)