update build process to use only Debian/buster packages
This is a major overhaul of the jekyll build process for f-droid.org. It updates the base OS from Debian jessie to Debian buster. It stops using jekyll/ruby packages from gem and instead uses only Debian packages. I backported a number of packages to Debian/buster to achieve this. It also updates a lot of Ruby packages so that there are no known security issues.
This will make it a lot easier for people to work on the website, since they can do it locally on Debian/buster without knowing Ruby/Gem. Those that know Gem can use it still. It will also make localization work a lot better due to improvements in po4a. This uses po4a from git until the last bugs are fixed in po4a, and that version is uploaded to Debian and backported to buster.
Lastly, it uses rolling releases of jekyll-fdroid from git, instead of requiring tagged releases. This was always a pain point when fixing issues in jekyll-fdroid. We might want to peg jekyll-fdroid to a commit here if we want to rebuild old versions of the website, but that doesn't seem worth much, and would still be possible to do by manually setting the jekyll-fdroid commit.
This also uses jekyll-polyglot from git to make it easier to work with the fork of it that hopefully improves the memory usage when building many languages.