We've noticed that our uwsgi queues are filling up and a lot of requests
are being made to robots.txt which ends up 500/503 erroring. Add a
robots.txt file which allows crawling of our lists and archives with a
delay value in hopes this will cause bots to cache results and not fill
up the queue with repetetive requests.
Change-Id: I660d8d43f6b2d96663212d93ec48e67d86e9e761