Add server block customizations

This change allows users to add a file `/etc/nginx/conf.d/includes/mydomain.com.conf`, the contents of which will be loaded into the server block for `mydomain.com` using an nginx `include` directive. Naturally, the `include` is added only if the file exists.

As with the YAML customizations, this puts the user in charge of making sure that the configuration actually works (`sudo nginx -t` is a good way to find typos).

Maybe there should be a documented way of triggering the web_update—I've been making edits to a dummy alias, but maybe there's a more elegant way.