[BUG] N+1 Problem on `/v2/classes` endpoint

[calumbell]

Bug description

The recently installed Django Debug Toolbar highlighted an issue on the /v2/classes that surface while I was exploring another issue:

### SQL queries from 1 connection
- **default** 19.06 ms (135 queries including 125 similar and 113 duplicates )

Classic N+1 behaviour. However on closer investigation it doesn't seem quite the same as other N+1 errors we have encountered on the server, where the problem is clearly triggered by a specific FK being incorrectly joined or prefetched.

SELECT "api_v2_characterclass"."name",
       "api_v2_characterclass"."desc",
       "api_v2_characterclass"."document_id",
       "api_v2_characterclass"."key",
       "api_v2_characterclass"."subclass_of_id",
       "api_v2_characterclass"."hit_dice",
       "api_v2_characterclass"."caster_type"
  FROM "api_v2_characterclass"
 WHERE "api_v2_characterclass"."key" = 'srd_warlock'
 LIMIT 21
125 similar queries. Duplicated 9 times. 

Steps to Reproduce

1. Fork the staging branch of the Open5e API. Install deps.
2. Visit the /v2/classes endpoint in a web browser
3. Investigate the Django Debug Toolbar