Make sure tails.net is available over IPv6

Originally created by @usul on #12113 (Redmine)

The internet is moving from v4 to v6. making the website and some mirrors available over v6 would probably help some people bypass ipv4 only censor tools.
For the record my mirror is ipv6 accessible.

Related issues

• Related to #16767 (closed)
• Blocked by tails/sysadmin#14588