archweb: tweak our rate limits

This tries to address two issues we have:

• Some folks retrieve the mirror status json every 4/5 seconds leading to ~ 8GB of bandwidth used per day. Now they will get banned by the nginx-limit-req jail of fail2ban
• Lower rate limits as they are too high, in reality loading an archlinux.org page has one request go to the backend. So the 20 requests limit is way too high. Lowering this has catched some silly wordpress/vulnerability scanners and they also get banned now by nginx-limit-req jail.