I did some clean-up / resync on the planet.gnu.org setup

• Fix issue with newer https websites (SNI)
• Re-sync Debian base config, scripts and packaging, update documentation; the planet-venus package is still in bad shape though, it's not officially orphaned but the maintainer is unreachable AFAICS
• Fetch all Savannah feeds using https
• Update feeds with redirections, which seem to mess-up caching