[BUGFIX] GameContentSerializer now inherits from ModelSerializer

[calumbell]

Description

This PR fixes an issue where unnecessary URLs were being generated for some of our find many endpoints (ie. fetching 50 Creatures caused about 600 calls to url() according to cProfile) taking up a significant amount of server time.

The issue was there GameContentSerializer, the abstract class inherited by the vast majority of our Serializers, inherited from the HyperlinkedModelSerializer, which always generates a URL when it is called, whether or not the serializer even returns a URL, or if it makes sense for it to!

The fix involved the following steps:

1. Replace HyperlinkedModelSerializer with ModelSerializer in GameContentSerializer's inheritance chain
2. Remove "url" from the "fields" array in serializers that inherit from GameContentSerializer.
3. Update our test suite to account for the removal of the "url" field from many serializers

Related Issue

Closes #896

How was this tested?

• Spot checked on local Django development server
• pytest (all tests passing, once cases updated to reflect new shape of API response)

[calumbell]

Closing and re-opening to re-run PR validation