Make the GROUP_CONCAT aggregate function not repeat values (closes #3223975) Work on sets instead of arrays, so if the same value appears twice it's not repeated in the concatenated output. This patch handles the postgresql and sqlite backends, mysql is left alone at this point (seems doable, but I don't have time or motivation to fix and test it).

raise NotImplementedError("Cannot migrate such an old version. Use intermediate Cubiweb version (try 3.16.x)")