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/policies/ip_ban_policy.rb

@@ -7,11 +7,11 @@ class IpBanPolicy < ApplicationPolicy
     user.is_moderator?
   end
 
-  def destroy?
+  def update?
     user.is_moderator?
   end
 
   def permitted_attributes
-    [:ip_addr, :reason]
+    [:ip_addr, :reason, :is_deleted, :category]
   end
 end