Don't enable IPv4 *only* on nginx.

The current configuration sample files only enable IPv4 by default, making the
server inaccesible for many remote hosts (and an increasing amount every day).

Enable 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.
Merge branch 'nginx' into 'master'

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