[WIP] Django 2.0 support

[maiksprenger]

I'm working on a project with Django 2.0 and noted that barbeque does not yet support Django 2.0. I was hopeful and thought it'd be easy to get it to work with Django 2.0 and started a pull request. But unfortunately django-cms is not yet compatible with 2.0, and hence the tests fail.

I hope that my work still provides a bit of value, and hence opened this PR so you can pick any commits you deem useful, @stephrdev.

FWIW, I think the non-CMS parts of barbeque should work with Django 2.0 just fine. I'll report if I discover otherwise.

[stephrdev]

Hi! Thank you for the contribution. Do you think we can add a pytest marker for the cms related tests and skip them in Django 2?

[maiksprenger]

@stephrdev Unfortunately that won't be easy, because we'd have to prevent any imports from Django CMS. It breaks on collection when importing CMS's page model. It would mean removing cms from INSTALLED_APPS etc. At that point we could almost have two separate projects, one with Django CMS as a dependency, and one without.

[stephrdev]

Alright. Lets hope that Django 2 support lands in Django-CMS soonish.