cubicweb #2807779 make `created_by` an inlined relation [in-progress]

The role/cardinality allow it. It would make it usable in multicolumn unicity constraints.

prioritynormal
typeenhancement
done in<not specified>
load0.200
load left0.200
closed by<not specified>
patchmigration for created_by [rejected][schema/base] created_by is now inlined (closes #2807779) [rejected]