Django 4 upgrade

[marksweb]

I've made a start on moving the project on to django 4.1

It doesn't support Postgres 10 which the github actions were still using - so I'm assuming that's also still the version in use in production. It looks like we can move straight to the latest - 15.1. I've setup locally with this and got everything running.

The other issue is that django-markdown-deux hasn't been updated since 2014. So I've forked it & brought django 4 support - and I might end up being the new maintainer of that so we don't need to install from my fork.

I've got some tests to fix, but otherwise everything looks good so far; https://github.com/marksweb/djangogirls/tree/feat/django4

[marksweb]

Got a bit delayed with this after starting a new job in February. Picking it up again with django 4.2