reduce our fail2ban usage

* stop blocking IPs based on their HTTP requests. This was originally
  added to frustrate bot signups but is problematic for a few reasons:

  * we block IPs that submit the sign up form 3 times in 10 minutes,
    which occasionally catches legitimate users (thanks NAT!)
  * if the request arrives via fastly then we block the fastly IP
    instead of the end user. That's Bad.
  * at our log volumes, fail2ban is very CPU intensive - it often pegs 1
    core

* this removes the monitoring of HTTP log files. The only thing left is
  monitoring the SSH logs and banning IPs that fail SSH auth too many
  times.