Allow nginx port to be customised via the GUI

[finnjsmith]

Currently the only way to customise the port is to manually amend /Users/finnjsmith/Library/Application Support/Herd/config/nginx/herd.conf and change this line:

server {
    listen 127.0.0.1:80 default_server;
    ...

to something like:

server {
    listen 127.0.0.1:8080 default_server;
    ...

I feel like this should be a customisation option offered via the GUI. In my case I had another application that required port 80 to be free, and it wasn't the easiest to find how to change Herd's default nginx port.