Closes: #17936 - GFK serializer field

[bctiemann]

Closes: #17936

[jeremystretch]

I found two serializers with generic foreign keys where GFKSerializerField is not being employed, presumably due to variations in the resolution logic.

• ObjectChangeSerializer.changed_object: Suggest we replace the SerializerMethodField with GFKSerializerField, and expose object_repr as a regular read-only field. IMO this cleaner than the current fallback behavior.
• EventRuleSerializer.action_object: I don't think the custom logic under get_action_object() is necessary anymore as there is a now a Script model: The automatic resolution should work.

[bctiemann]

Added those two; tested successfully, works nice. Added ObjectChangeSerializer.object_repr as a read-only CharField.