Make nginx redirect server an example

This is unusable with the install script as you can’t pass additional
domains to it. Also makes the assumption that you’re using www. as the
canonical domain name, which might not be the case if you were running alaveteli as as e.g. foi.example.org.
Make setting the server_name a manual step

Interferes with the install script and the server will work without it providing it is the only server available
Don’t assume alaveteli is the default server

Can either be added manually (or by the install script) or nginx will default to this server if there are no others available