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`

Yea, it looks like you linters just did a bit of cleanup and this was wrong previously, but if it's easy to fix, may as well do it while you're in here.