Which ZIMs do we wanna backup? Where?

Currently ZIM files are not backup. If something bads happens to the server (`SRV-B` ATM), we loose the files and do not have a backup.

Production ZIMs of Kiwix collection are mirrored so this was deemed to be sufficient so far.

This means that:
- staging (old dev) and "backup" CMS ZIMs are not backup
- any other CMS collection is not backup (branded apps, endless, ...)

While it seems to be an acceptable risk on the short term, recent discussions with @kelson42 and @rgaudin kinda prove it is time to change that.