This once again enables using the provided dev scripts to
set up a fresh development environment with just Python
installed - no extra config and tweaks needed.

- Split dev configs into per-database files
- Simplify scripts in "tools" and use per-database configs
- Allow overriding DJANGO_SETTINGS_MODULE for dev scripts
- Update .gitignore for more venv aliases and Django stuff
- Update links for Django 2.2
- Misc comment cleanups