Nginx config

In the guide I've [linked it](https://github.com/mysociety/alaveteli/commit/031e7acb9ecfc3934b8ee64b651af289afd9cac5#diff-52550ceabfa1199ba2e32a5869ba57a0R653) to `/var/www/alaveteli`, but `/srv/alaveteli` is the "correct" location. I think we should do as much as we can to make transitioning the app to do things The Right Way will make efforts like [including alaveteli in Debian](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=742931) much more likely.