4559 config context rendering

[lampwins]

Fixes: #4559

This implements a new queryset manager for the device and virtual machine models which includes an annotate_config_context_data() method. This method uses a dynamically generated subquery with the JSONBAgg function to annotate the list of config context json objects directly on the device and virtual machine querysets.

There are a couple of things to note:

• Django has included the postgres JSONBAgg function for a while but in 3.2 it includes support for aggregation ordering. My implementation includes a custom subclass which implements the same until we move to 3.2. Ordering is a critical component of being able to do this as a subquery.
• The original get_for_object() method is still present and actively used. This is still needed to query for the actual ConfigContext instances for a given object, and in certain cases where the annotation is not available.
• Because the annotation method is optional and not added to the queryset by default, the existing ability to exclude config context data from the API response is still in place.

[jeremystretch]

Great work! And yes, much faster. Just had a couple minor points. If you don't feel the need to act on either, I'm fine with merging this as-is. 👍