Use Express 4.10, which has res.sendFile, which we've been using in the ImageFile controller for a while now (and wondering why it wasn't working)....
Back to generic error message. Will think about proper error reportage later.
1. Rename CouchDB views from by_x to x_by_y, to better document what they actually return.

2. Implement ImageFile.getLatest and ImageRecord.getLatest, using the (newly spiffed-up) path_by_create_date CouchDB view.

Don't really need the latest.jpeg symlink any more. Perhaps it will go away now.