ip bans: add hit counter, deleted flag, new ban type.

* Make IP bans soft deletable.
* Add a hit counter to track how many times an IP ban has blocked someone.
* Add a last hit timestamp to track when the IP ban last blocked someone.
* Add a new type of IP ban, the signup ban. Signup bans restrict new
  signups from editing anything until they've verified their email
  address.

app/controllers/ip_bans_controller.rb

@@ -19,9 +19,10 @@ class IpBansController < ApplicationController
     respond_with(@ip_bans)
   end
 
-  def destroy
+  def update
     @ip_ban = authorize IpBan.find(params[:id])
-    @ip_ban.destroy
+    @ip_ban.update(permitted_attributes(@ip_ban))
+
     respond_with(@ip_ban)
   end
 end