Fix KeyError in nested serialization of ConfigContext, and add ObjectChange API test, add ObjectChange unique_together

[glennmatthews]

Closes: #3885

See also #3872 (comment)

What's Changed

Change self.fields.pop("dynamic_groups") to self.fields.pop("dynamic_groups", None) as in the case of nested serialization of a ConfigContext (such as within an ObjectChange), M2M fields such as dynamic_groups are already automatically excluded from the serializer.

Add missing API test cases for the ObjectChange REST API read-only endpoint and confirm that they catch this specific error.

Add a unique_together on ObjectChange model of (time, request_id, changed_object_type, changed_object_id) both for general purpose data correctness but also to provide an appropriate natural_key for this model (fixes #3885).

note: [request_id, changed_object_type, changed_object_id] alone is not a sufficiently broad unique_together as a single bulk-action request may result in changing the same object multiple times. Thank you unit tests!

Add basic model test case for ObjectChange to exercise its natural-key and composite-key construction.

TODO

• Explanation of Change(s)
• Added change log fragment(s) (for more information see the documentation)
• n/a Attached Screenshots, Payload Example
• Unit, Integration Tests
• n/a Documentation Updates (when adding/changing features)
• n/a Example Plugin Updates (when adding/changing features)
• Outline Remaining Work, Constraints from Design

[jathanism]