Django 2.0 support (dropping Django 1.8 support ?)

[smendes]

We are using both Wagtail and ModelCluster on a project and after trying to run Django 2.0 Beta 1 I found some issues in ModelCluster.

I've made a few changes locally to make it run:

• field.rel -> field.remote_field (in django.db.models.fields.Field)
• field.rel.to -> field.remote_field.model (in django.db.models.fields.reverse_related.ForeignObjectRel)

Looking through the issues I've found your answer in Wagtail's issue #3599. I was wondering if there are any plans to drop the support of Django 1.8 for ModelCluster and move forward with those changes.

And if that's the case I can provide a PR of those changes if you want.

[gasman]

Thanks for investigating this, @smendes! Yes, I think now is the time to drop Django 1.8 support in favour of 2.0. A PR would be very welcome.

[smendes]

PR opened #86
To make it work it depends on an update of django-taggit.

Also worth mentioning I've added an empty implementation of _apply_rel_filters that should be compatible with all versions but that can probably be done much better.

[gasman]

Completed in #86.