Thumbnailing.

Most notably, uploading a file will now generate a thumbnail (and a
"medium" size image).

Also tried to clean up use of storage.  I'll revisit this again later,
but the intent was to make it easier to switch from file storage to
something like S3 or mogile or whatever in the future.  The biggest
change was to keep storage access out of the db model; Art no longer
knows anything about storage whatsoever.