Initial sketch of bug #282, aggregate cell data export.

I believe this mostly accomplishes the "hourly exports of changes to Cell table" issue, in very broad brush strokes. I haven't managed to test it against S3 live yet and I expect the fields will change some and possibly need preprocessing before export, but I'm posting this for feedback / review now.
I'd just re-use the assets bucket for this. It contains publicly accessible data and we the image tiles are all under a /tiles folder, so we can just use a different top level folder.