Store the files using consistently-hashed subdirs.

The idea is to allow the external loading app to prepare the html and store the originals in S3 by changing the variable elements of the document storage path such that it uses a completely reproducible hash scheme (consistent hashing!  Huzzah!) instead of a system-generated (uu)id.  This lets us offload the processing/storage and enables the use of something like upsert or activerecord-import (my favourite) to bulk load the files without overloading the online system with s3 and tika data processing.