* 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.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
