add Onion-Location header - offcial F-Droid .onion service

This will make the ".onion available" button in Tor Browser show up and point users to: http://fdroidorg6cooksyluodepej4erfctzk7rrjpjbbr6wx24jh3lqyfwyd.onion/

The onion service is fully scripted and runs on a vps payed for with our open-collective donations. The vanity .onion address was generated by @eighthave.