Uploading files individually to Dropbox is flaky, and the unique URL per
file complicates data/MANIFEST.
While we're at it, start carving aside a directory structure for keeping
data for the last few releases.
[rebuild] to make sure github actions can download everything
showed this is faster than downloading the uncompressed files, but of
course it probably depends on network speed vs CPU. #326
Also cut over the updater to use gzipped files using flate, instead of
single-file .zip's. [rebuild]
