Remove explicit simplejson import

Django 1.5+ drops support for Python 2.5 so we can assume that json is
available: https://docs.djangoproject.com/en/dev/releases/1.5/#django-utils-simplejson
Convert Leaflet.live field to NullBooleanField

Our data dump has created the 'live' field as a BooleanField, not an
IntegerField.

Accordingly, amend the models and view to be a BooleanField.
Fix HttpResponseRedirect: leaflet url lookup expects primary key

This URL lookup expects 'pk' not 'object_id'. Fix.