Modernizing the project + bug fixes

• Switched to Poetry (https://python-poetry.org/) + more cleanup
• Switched to pytest and pytest-django
• Drop support for pre-Django 1.10 style MIDDLEWARES
• Switch to old style url to path in urls.py
• Added Black and re-formatted and linted all the code
• Added isort and sorted all imports
• Bugfix: Add rule in admin was throwing an exception
• Handling "unknown" values in X-Forwarded-For header; introduces IPRESTRICT_USE_PROXY_IF_UNKNOWN setting
• More cleanup