This prevents "weird" objects from lingering around. Now, if a user is
deleted, their tracks are also being deleted - which is good, especially
for the user's privacy.

For a more "graceful" closing of the account, we could implement a
strategy that first re-assigns the tracks to a different owner, and then
deletes the account. But that may only happen with the consent of the
user, and it is a work for future improvements.