Bugfix (API V2): Added missing Serializers to CreatureSerializer

[calumbell]

This PR fixes a bug where the environments and languages fields of the /creatures endpoint could not be filtered via query parameter.

eg. v2/creature/?environment__fields=name and v2/creature/?languages__field=name had no effect on the API response.

This was because the LanguageSerializer and EnvironmentSerializer were not included on the CreatureSerializer, so these fields did not correctly inherit the logic defined in the GameContentSerializer abstract class for managing filtering nested fields.

Now, the query parameters for a creature of the form /?fields=languages,environments&languages__fields=name&environments__fields=name will generate a response of the form:

{
    "languages": [
        {
            "name": "Common"
        },
        {
            "name": "Draconic"
        },
        ...
     ],
    "environments": [
        {
            "name": "Arctic or Tundra"
        },
        {
            "name": "Caves"
        },
        ...
    ],

[calumbell]

Ah looks like, this is failing tests. To be expected, as it does change what is returned by the API when no parameters are provided. Should be a simply a case of updating the tests to match the new schema

[calumbell]

(Tested updated and passing, good to go!)