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/users_controller.rb

@@ -59,9 +59,11 @@ class UsersController < ApplicationController
   end
 
   def create
+    requires_verification = IpLookup.new(CurrentUser.ip_addr).is_proxy? || IpBan.hit!(:signup, CurrentUser.ip_addr)
+
     @user = authorize User.new(
       last_ip_addr: CurrentUser.ip_addr,
-      requires_verification: IpLookup.new(CurrentUser.ip_addr).is_proxy?,
+      requires_verification: requires_verification,
       name: params[:user][:name],
       password: params[:user][:password],
       password_confirmation: params[:user][:password_confirmation]