Django has made the `on_delete` argument to `ForeignKey` mandatory. Until now, this has defaulted to `CASCADE`, which makes it easy to unintentionally delete things that shouldn't be deleted. Making it a required argument forces developers to think about each model and how it should behave. This change explicitly sets `on_delete` for every relation we have. Most of these are `CASCADE`, but the diff data ones are `SET_NULL` because those are effectively a one-to-many relation, and we don't want to delete diff data that may be shared by other diffs. Testing Done: Ran unit tests.