Initial testing fixes

[nickmoreton]

This adds github actions and alters the tox testing.

I thought this is a good place to start with getting the existing tests passing and implementing some CI/CD checks.

There are some changes I needed to make because I've assumed the Wagtail version support is >=2.11, <2.14 and Django==2.2. I have also only added pipeline testing for python 3.7 - 3.9 for the moment.

Additionally

• local python version 3.7
• I've not focused any attention to the code other than to get the tests passing

Next steps/thoughts:

• I could bump the Wagtail and Django tox versions but without making further changes to the code the test fail so I think this is a good first step.
• Create another PR to test more versions of Wagtail and Django (as well as python versions)
• Leave Modernise for latest Wagtail and Django #474 in place as a reference to work on the next PR's but not to be merged.
• Work on a development roadmap.
• Any other thoughts welcome 😃

[nickmoreton]

@JamesRamm thanks for merging this 👍