cubicweb #893940 finish undo support [validation pending]

  • undo of entity update
  • better handling of entity add undo (conflict detection) ?
  • replace config by a simple undo-support= yes or no ? would probably be better since we can't properly tell what action should be undoable or not (should be all or nothing in a transaction)
appeared in<not specified>
done in3.15.0
load left0.000
closed by#8271:486386d9f836, #8269:9747ab9230ad