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.

Ran unit tests.