Set up project tooling (for Django / Python)

Mostly configuring black, isort and flake8 and removing old config files.

Changes

[ ] Configure black via pyproject.toml (and point to config in .vscode settings)
[ ] Configure isort via pyproject.toml (and point to config in .vscode settings)
[ ] Configure flake8 via setup.cfg (and point to this config in .vscode settings)
[ ] Remove old .flake8 and .vscode settings
[ ] Fix linting, type checking ... etc in model files.