ForeignKey accepts an on_delete argument that says that should

happen when the object on the other end of the relation is deleted.

Historically, Django has defaulted to CASCADE, which deletes the

object on this end of the relation, but there are other options

available.
In order to prevent unintentional deletions, Django 2.0 requires this

option, and Django 1.11 emits a deprecation warning if it's not set.

This change goes through all ForeignKey fields in Djblets and sets it

to the historical default of CASCADE.

Unit tests on Djblets and Review Board pass for Django 1.6.