Debian/buster update

This is a big overhaul of this code so that it runs on recent Jekyll versions, and works with only Debian packages from buster and buster-backports. It removes quite a bit of cruft, and moves some code to fdroid-website, where it will be easier to maintain. Details in the commit messages.

@Bubu @uniqx I know you don't know Ruby/Jekyll, but I barely do. So any review would be helpful.