1589 feature flags cannot be assigned when creating an user from the admin

@Kartones CR this, please. This is related to #1589, but doesn't really solve any problem, since it already worked for me before this changeset. As you see now I won't set_relationships_from_central unless save worked, in order not to hide other problems. Changes at `cartodb_central_synchronizable.rb` are because I was adding an array to the array, instead of its elements, but since that association is not used in that request it had no effect.