Update to support wagtail 2.8 and django 3.0

[lingster]

Hi, Could someone kindly review this PR and merge to master?

Many thanks, Ling

[gannebamm]

It seems this project is dead? Without django 3 support it wont be useful.

[lingster]

@gannebamm : I've not heard back from any of the maintainers. I've dropped them an email and will wait to see their reply. If you are using pipenv you could install the version in my fork by adding this to your Pipfile: wagtailstreamforms = {git = "https://github.com/lingster/wagtailstreamforms"}