Make sure tails.boum.org is available over IPv6

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 #14588 (closed)

Original created by @usul on 12113 (Redmine)

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