Fix a few django deprecation warnings

[rpkilby]

No description provided.

[carltongibson]

Great. Thanks!

[xordoquy]

I think those cases could have been spotted if the models were in a models.py file.
The went under Django's check radar because of that.
Still need to figure whether it's ok or not.

[carltongibson]

Still need to figure whether it's ok or not.

I not sure I follow you. 🤔

Docs:

Deprecated since version 1.9: on_delete will become a required argument in Django 2.0. In older versions it defaults to CASCADE.

What's not right? (Or maybe not right?)

[xordoquy]

I though that was required in 1.10.x

[carltongibson]

But we want it right? (Sorry — I think I need one-more-☕️ this morning...)

[lovelydinosaur]

Great stuff!
Not sure I followed @xordoquy totally there, but we can always follow that up separately.