Merge pull request #13 from open-oni/feature/leaflet-1.9.4

Update Leaflet to 1.9.4; Fix font-size

Update urls.py

- Switch to `django.urls` rather than `django.conf.urls` which is
  deprecated and will be removed in Django 4.0

Update default branch name to main

Update compatibility notes

Merge pull request #8 from open-oni/leaflet-update

Update Leaflet to latest release 1.5.1

Merge pull request #3 from open-oni/feature/aggregate-cities

Feature/aggregate cities