Use DataDog's ddtrace package for Datadog APM integration

[bittner]

Currently, we use the seemingly abandoned django-datadog package (source code) for integrating with Datadog.

As a result of the 2015 and 2017 discussions, it turns out that we should replace this by Datadog's ddtrace package, which integrates into Datadog's (non-free) APM, conveniently.

• Django integration (official docs)

Additional functionality (e.g. setting deployment markers?) may be available with Datadog's datadog Python package.