Add Wagtail 2.0 compatibility

[loicteixeira]

Edit: Checklist moved to PR.

[dgkohn]

We have released https://github.com/springload/wagtailmodelchoosers/tree/v2.0b1 for now, which is working with Django>2.1,<2.2, Wagtail>2.3,<2.4 and django_filter>=1.1,<2.1.