Simplify IPv6 firewall rules

Originally created by @intrigeri on #7668 (Redmine)

This has been discussed on tails-dev@ in July, 2014 (firewall rules thread). The proposed patch was authored by Jacob Appelbaum.

Feature Branch: feature/7668-simplify-IPv6-firewall-rules

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information