Add support for soft-deleting

[raderio]

In some applications, you don’t want to, or you are not allowed to permanently remove a record from the database. But you still need to remove or hide records which are no longer active. One example could be a user account which you want to keep because it is linked to other business objects which are still used.

Typical story: after an entity is marked with deleted=true, it should not be visible anywhere, except specially chosen places.