Allow internal IPs access to demos with basic auth.

This should allow the QA test infrastructure to run against the demo servers even
if they have basic auth on.
Unfortunately it's a little more complicated than this: from apache's perspective *everything* is coming from that subnet, because the zlb internal IP is in this range. Based on the `Deny from 10.8.81.217` above this (IP based on `zcat /var/log/httpd/www.mozilla.org/access_demo*.gz | awk '{ print $1; }' | uniq` from the dev server; not sure were this is documented), but that is fairly brittle so we may want to double check with webops to see what the odds are of this changing without us knowing about it.