Remove Content Store caching

Remove all use of the `redis_content_store` caching module, replacing `content_cache` calls with standard `cache` directives.

ContentStore's reference objects don't play nicely with letting Redis expire keys when needed.

This change will slow response times because of adding new SQL queries (to get objects so that we can use them in a cache key), but we'll profile that once it is deployed and come up with solutions for any hotspots that pop up.