Allow to indgest ZIM popularity

[kelson42]

We want to be able to weight the result of a library search based on there popularity. The popularity is defined by the number of downloads (proportinally to the other ZIMs).

That means that we need:

• To indgest the number of downloads of all ZIM base on https://stats.kiwix.org
• Compute a popularity rate for each ZIM file over time (doing this time to time)
• Apply the popularitz to each ZIM file in the DB
• Export the popularitz in the XML (or ultimatively in the OPDS).

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be now be reviewed manually. Thank you for your contributions.