Allow selective display of ZIM files on library.kiwix.org

We've had at least two instances in recent months of content publishers balking at the availability of their content at library.kiwix.org, while being committed to offline distribution via the apps.

Not sure where this issue should go, but I think we should somehow be able to flag some of the content as "web-hidden" so it only appears in the app catalogues.

Start by tracing how ZIM files are selected and displayed in the library.kiwix.org catalogue, then determine where a web-hidden flag could be represented without affecting app catalogues. Done means publishers can keep content available for offline apps while selectively excluding it from the website.

Written by the indexing model from the issue text.