* FIX: Posts can belong to hard-deleted topics
This was a problem when serializing deleted posts because they might
belong to a topic that was permanently deleted. This caused to DB
lookup to fail immediately and raise an exception. In this case, the
endpoint returned a 404.
* FIX: Remove N+1 queries
Deleted topics were not loaded because of the default scope that
filters out all deleted topics. It executed a query for each deleted
topic.