Django 2.0 support

[Woodzyowl]

Django 2.0 requires that foreign keys have explicit deletion behavior. This adds in said explicitness. Default behavior is cascade, so I kept it as the default.

[bentleycook]

LGTM

[mrhwick]

While we're in there, are you sure that we don't want to add signals to mimic the cascade on_delete behavior when the content_object is deleted?

[mrhwick]

Otherwise, LGTM

[Woodzyowl]

Dammit. I forgot that these two jokers still had access to this repo 😛
And no, I'm not sure about that at all. My only current objective is to eliminate the errors being thrown when I try to run the server. Make it work -> make it work well -> make it work fast

[bstrdsmkr]

Care to squash those commits?

Otherwise, LGTM =)

[pierce-m]

We squash on merge now