Don't enable IPv4 *only* on nginx.

I came across this issue when initially setting up gitlab and fixed it locally. Today, helping someone else set up their own instance, I came across this issue again:

The sample nginx configuration files disable IPv6 by default, making the server inaccesible unless the remote node has native IPv4. IPv4->IPv6 transition mechanisms don't help if the server have proper IPv6 DNS records in-place, which was the case for both of my servers.

This branch enables IPv4 and IPv6 by default. Older servers with no external IPv6 connectivity will not fail since they'll have a local-link IPv6 address to bind to anyway.

See merge request !231