FIX: TopicLinkClick: do not log IP of logged in users

db/migrate/20180521190040_allow_null_ip_topic_link_click.rb

@@ -0,0 +1,10 @@
+class AllowNullIpTopicLinkClick < ActiveRecord::Migration[5.1]
+  def up
+    begin
+      Migration::SafeMigrate.disable!
+      change_column :topic_link_clicks, :ip_address, :inet, null: true
+    ensure
+      Migration::SafeMigrate.enable!
+    end
+  end
+end