Avoid using deleted products in Stock Planning

[pylipp]

https://trello.com/c/DKuBXKJ8

For testing, log in as user of base 1.

1. comment the fix
2. clone the "grouping" prouduct for e.g. trousers: insert into products (name, category_id, gender_id, sizegroup_id, camp_id) values ("Trousers", 2, 1, 1, 1)
3. delete the "grouping" product for e.g. trousers: update products set deleted = utc_timestamp() where id = 1174
4. reload, and click Bottoms -> Trousers -> Boy. The overview is now empty because the URL uses the deleted product 1174
5. activate the fix and reload Stock Planning
6. click Bottoms -> Trousers -> Boy. Overview shows two boxes, and the URL correctly uses the new product ID