Issue #1985 - track email open rate

Work in progress, updating #2015.

Use [django-gabeacon](https://github.com/jarekps/django-gabeacon) to serve a 1x1 transparent gif pixel in emails. When the 1x1 gif is served, we send a request - via celery - to the GA collection service using the [Measurement Protocol](https://developers.google.com/analytics/devguides/collection/protocol/v1/).