Add fail2ban config to block bots that attempt to sign up too often

This is an experiment in using fail2ban to stop bots signing up too many fake accounts.

After 10 signup attempts in 10 minutes from the one IP triggers, a 6 hour ban is put in place using iptables.

What does everyone think about the limits? Too strict? Not strict enough?

Is fail2ban the right option? A rails-centric approach would be possible be storing the IPs used to submit sign ups in our app Db for 24 hours.

@benhoskings - can you work your magic to DRY up these deps?