Fix migration restoreReferencesOrder removes references

The restoreReferencesOrder iterates over all UIDs in `obj._relatedItems`
to recreate `obj.relatedItems`. That way the references from `obj._backrefs`
are removed.

The fix removes restoreReferencesOrder and adds the `beforeChange_relatedItemsOrder`
method to fhe ReferenceMigratror class. That method stores Archetype relations as
target uids on the old archetype object to restore the order later.
Because all relations to deleted objects will be lost, we iterate
over all backref objects and store the relations of the backref
object in advance.