Get rid of `if` usage in nginx config

[chiel]

We tried to get rid of the newly-introduced if statement inside the cors.conf in order to avoid if, since it is generally discouraged by nginx.

With the help of @stevenjm we came to the following:

# /etc/nginx/location.d-enabled/cors.conf
# Allow CORS if environment variable is set
add_header 'Access-Control-Allow-Origin' "http://127.0.0.1:8888" always;
add_header 'Access-Control-Allow-Credentials' 'true' always;
add_header 'Access-Control-Allow-Methods' "*" always;
add_header 'Access-Control-Allow-Headers' "*" always;
add_header 'Access-Control-Expose-Headers' "*" always;

# /etc/nginx/conf.d/default.conf
map $request_method $rewrite_target {
    default /index.php;
    OPTIONS /NGINX_INTERNAL_HANDLE_OPTIONS;
}

server {
    listen 80;
    listen [::]:80;
    server_name  localhost;

    root    /opt/project/public;
    charset UTF-8;

    include /etc/nginx/location.d-enabled/*.conf;

    error_page   500 502 503 504  /50x.html;
    location = /50x.html {
        root   /usr/share/nginx/html;
    }
    
    location / {
        rewrite ^ $rewrite_target last;
    }

    location = /NGINX_INTERNAL_HANDLE_OPTIONS {
        return 204;
    }

    location ~ \.php$ {
        fastcgi_pass   unix:/var/run/php-fpm.sock;
        fastcgi_split_path_info ^(.+\.php)(/.*)$;

        include        fastcgi_params;

        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param DOCUMENT_ROOT $document_root;
    }
}

The map could be moved into the cors.conf and that file could, for example, live in the conf.d directory, but I'll leave that to you guys to figure out.