[django] Update sentry configuration

[nemesifier]

Update the sentry configuration to use the latest configuration method (drop raven and use sentry_sdk).

• configuration for django: https://docs.sentry.io/platforms/python/django/
• configuration for celery worker (will be needed with connections module of openwisp-controller: https://docs.sentry.io/platforms/python/celery/

[pandafy]

configuration for celery worker (will be needed with connections module of openwisp-controller: https://docs.sentry.io/platforms/python/celery/

@nemesisdesign I don't think we need to anything extra for monitoring celery workers.