Fixed link to parent WP in the WP table

https://www.openproject.org/work_packages/14707
Because it won't work on the client side. There are two different scenarios:

1. `if model_type == work_package` -> link to the current work package
2. `if model_type == parent_work_package` -> link to the parent work package

I don't now how could we tell on the client which of these should happen without differentiate between `work_package` and `parent_work_package`.