Dark mode and UI cleanup

Changelog:

• Switch from pipenv to poetry for dependency management
• More robust parsing of systemctl output on Ubuntu
• Dark mode UI
• Better UI for mobile devices

If you're upgrading from 0.2.x to 0.5.x:

• Activate your venv and:
  • run poetry install to add and update dependencies
  • run python manage.py migrate to update the DB to add new invite functionality and constraints
  • run python manage.py create_groups to add the default permission groups
  • Restart the service (if you used the provided systemd service: systemctl restart wireguard-web-ui)