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/application_controller.rb

@@ -163,7 +163,7 @@ class ApplicationController < ActionController::Base
   end
 
   def ip_ban_check
-    raise User::PrivilegeError if !request.get? && IpBan.is_banned?(CurrentUser.ip_addr)
+    raise User::PrivilegeError if !request.get? && IpBan.hit!(:normal, CurrentUser.ip_addr)
   end
 
   def pundit_user