When tracker is in maintenande mode return HTTP status code 503 (instead of the current 200 and 400 http status cods) (#11773)

* When tracker is in maintenande mode, return HTTP status code 503 instead of the current status of 200 or 400

This will make our return code consistent with the UI which also returns 503 while in maintenance mode

 Learn more about starting a maintenance window: https://piwik.org/faq/how-to-update/faq_154/

* Update TrackerResponseTest.php

* Update TrackerResponseTest.php

added a new url parameter send_image=0 to disable sending an
image and instead response with a 204 code