FIX: Don't fail topic auto-close if privledges are lost

app/jobs/regular/close_topic.rb

@@ -4,6 +4,10 @@ module Jobs
     def execute(args)
       if topic = Topic.find_by(id: args[:topic_id])
         closer = User.find_by(id: args[:user_id])
+        guardian = Guardian.new(closer)
+        unless guardian.can_close?(topic)
+          closer = Discourse.system_user
+        end
         topic.auto_close(closer)
       end
     end

lib/guardian.rb

@@ -114,6 +114,7 @@ class Guardian
   alias :can_move_posts? :can_moderate?
   alias :can_see_flags? :can_moderate?
   alias :can_send_activation_email? :can_moderate?
+  alias :can_close? :can_moderate?
 
   def can_grant_badges?(_user)
     SiteSetting.enable_badges && is_staff?