fail2ban-like ip blacklisting mechanism

This adds a fail2ban-like IP blacklisting mechanism to x/84. If an IP makes a connection more than w times in x seconds, their IP will be blacklisted for y seconds. Any further attempts during the blacklisting window will increase their ban time by z seconds.

All options have defaults; just adding `[fail2ban]` to `default.ini` is enough to enable the mechanism. If you want to override them, however, here is an example:

````
[fail2ban]
ip_blacklist = 1.2.3.4 5.6.7.8  ; list of IPs to permanently ban
max_attempted_logins = 3  ; attempts per window before ban
max_attempted_logins_window = 30  ; seconds for window
initial_ban_length = 360  ; seconds to ban someone at first
ban_increment_length = 360  ; seconds to add for repeated attempts during ban
````