Partially fixes #9374 - Implement a custom paginator for DeviceViewSet to improve config_context load times

[kkthxbye-code]

Fixes: #9374

Running count on the annotated query for loading config_context is slow. The custom paginator removes the annotation before getting the count.

Testing with 35k devices loading the devices list via. the API improved load times from ~3200 ms to ~1600 ms.

This doesn't fix the underlying issue in that the query config_context annotation is slow in general and scales with the number of devices, regardless of number of records fetched.

[jeremystretch]

This doesn't fix the underlying issue in that the query config_context annotation is slow in general and scales with the number of devices, regardless of number of records fetched.

I don't know that we can consider this a bug frankly, since it's more of an optimization issue. The fix your provided here works great to address the most obvious issue, and IMO resolves #9374.

We should probably look into implementing this more widely across the application, as there are instances where other queries could be sped up by shedding annotations from their COUNT() statements. I'll open a separate housekeeping item to pursue that once this has been tested more widely.