Fix Library so that required archives can be downloaded from non-mirrored directories #288
Description
Due to a change at download.kiwix.org, downloading of custom_app archvies and other required content is broken. The new download location for this content does not have mirrorbrain features and the archives are no longer mirrored. I need to work around this, until kiwix/kiwix-js#846 can be implemented (possibly helped by kiwix/libkiwix#585 and kiwix/libkiwix#797).