Make sure that when we publish data, we don't just publish the latest census. We need to publish the latest census PLUS every census prior.

[justinpac]

• In other words, when census N is published, we create snapshot N of of every census that includes Census N-1, N-2, .... 0.
• In the SI data, we don't have every snapshot present all at once. Instead, only snapshot N is present at a given time. Snapshots N-1, N-2, .... 0 are contained in SQL backups zipped into files.
• Allow the user to indicate which censuses they want to export.