By default, `has_and_belongs_to_many` properly deletes the record in
the join table.
However, as the association is declared manually with a
`has_many / through`, it doesn't delete the joined record automatically.
As we also lack a foreign-key contraint on the join table, that means
a dangling record remains in the join table.
To fix this, let's declare it a proper `has_and_belongs_to_many`
association, which will let the join record be deleted automatically
on destroy.
71e1b6c973