Added heartbeat for pingback.

Added heartbeat for pingback.

The current implementation of the pingback feature sends new data on
the first page load after running the update.php maintenance script if
no ping has yet been sent for the wiki's current MediaWiki version.
There is no way by observing the event log to determine if a given wiki
is still operational, since no further pings are sent unless the
MediaWiki version on the wiki changes. Wikis that are created for
testing purposes or that have been decommissioned will continue to live
on in the pingback data.

This patch adds a monthly heartbeat ping. The structure of the heartbeat
ping is identical to the original ping. The heartbeat ping serves not
only to indicate that the wiki is still alive; it will send updated
information, so it will be possible to find out if any of the data, such
as the PHP version or memory limit, has changed since the last ping even
if the MediaWiki version has stayed the same.

Bug: T189785
Change-Id: Ia3077ed02e36eb6ad6ef0ae4d085ecaeb1547a52