Allow Internet Archive to archive etherpads

This augments the default of disallowing all robots to crawl etherpad sites
and specifically allows Internet Archive to archive pads on
etherpad.wikimedia.org. This was explicitly requested on BZ: 56893 as a
way to backup pads

Change-Id: Ie24e84757e491bec7654b251b59b02c6d5eb4ef1
BZ: 56893