Django 2.0 requires ForeignKey instances to have on_delete set.

Prior to 2.0, these defaulted to CASCADE, which would delete the

owning model if the object on the other end of the relation was deleted

within Django's ORM. In 2.0, they apparently wanted this to be explicit,

and 1.11 pretty heavily announces deprecation warnings in preparation

for that.
This change adds explicit on_delete settings for each ForeignKey in

the codebase, and updates the creation of ForeignKeys from a database

signature to provide a suitable default if not found in the signature.

Unit tests pass on Django 1.6 through 1.11, and on 2.1 with other

pending changes.