moving sites-enabled to /etc/nginx/sites-enabled

This is a small change that moves our sites-enabled directory to `/etc/nginx/sites-enabled`.

It does not remove the old directory `/edx/app/nginx/sites-enabled` which will no longer be used, it also keeps the sources in `/edx/app/nginx/sites-available`

I kind of like being explicit on the location, but since you already changed `{{ nginx_sites_enabled_dir }}` why not just leave this template as is?