Category is duplicated when deleted from the parent and added to the new parent.

[teddychoi]

To move a Category object from the parent to the new parent, the category should be discarded from the parent and added to the new parent. But when a category is deleted from the parent and added to the new parent, the created_at attribute is overwritten with the newer time than the deleted_at, and as a result, the category exists in both two parents. The converse situation is also true.
Here is a regression test.

def test_previous_change_applied():
    subs = SubscriptionList()
    category = Category(label='first-order')
    subcategory = Category(label='second-order')
    to_move = Category(label='to-move')
    category.add(subcategory)
    subs.add(category)
    subs.add(to_move)
    assert len(subs) == 2
    assert len(subs.categories['first-order']) == 1

    subs.discard(to_move)
    category.add(to_move)
    assert len(subs) == 1
    assert len(subs.categories['first-order']) == 2

To work around this, I have to save the SubscriptionList to the stage after deleting the category from the parent, reread the SubscriptionList, and add the category to the new parent.