Adminfrontend: When Deleting BoatTypes or Sports, don't cascade deletion, but keep boats to be reassigned to other sports

This goes both for boats when deleting boat types, and for boat types when deleting sports. At the moment, this functionality is kind of there already, as the boats stay in the database when a boat type is deleted (we still need to look if the same is the case with boattypes->sports). However, in the Adminfrontend, those orphan-boats are not displayed, so we need to check the case of a missing boat type, display the boats, and colour the row accordingly so that an admin can quickly see that the boat needs to be reassigned to a type.