Make it possible to mark services as ipv6-only

[puiterwijk]

This might not be all too useful for Fedora, but other instances might have IPv6-only services.

[ralphbean]

None of our apps would have it set to true, like you said. Do you know of another instance that wants this?

How do you plan to set the javascript var?

[puiterwijk]

An instance that uses this is https://puiterwijk.org/, that's what I built this for :).

I currently set the javascript var by serving a templated index.html.
Perhaps it would be a better idea to serve an is_ipv6.js, so that only that needs to be templated, version for that is in #28.

[ralphbean]

Closing in favor of #28.