Ensure Person/Audit has mapping model name, for inner nav count.

Per [72697008](https://www.pivotaltracker.com/story/show/72697008).  Audit count not loading.  Problem was traced to how `init_count` for Audits was guarded by an `allow_reading` check, which was incorrectly set because (after the new descriptor refactoring) it couldn't find a canonical mapping from Person to Audit.

Fixed by adding "Audit" to Person's related object, with note about this coupling.