Public server firewall configuration

Added section on firewall configuration.  This should prevent users (like me) from struggling to figure out why their servers aren't executing code.

This documentation applies to current stable release and older releases, and so probably should be backported at some point.
It also affects running the server externally because the server's firewall must allow loopback connections to 127.0.0.1.  I set up an external server recently and although I could access the interface, no code would execute.  I was confused for a couple of days before I figured out what was happening.

Should I make it more clear in my explanation that the *server* must be able to connect to itself (127.0.0.1)?