Track user engagement

Use pixel to track user engagement:

<img height="1" width="1" border="0" style="display: none;" src="http://tracking-domain.com/tracking202/static/eng.php" />

You may show it for some event (user interacted, user moved to second
stage of landing page, etc.).

Currently engagement is only recorded in database
(202_clicks_record.click_reviewed). In near future it would be possible
to see engagement rate among with CTR on "Group Overview" page.