Add option for sharing info about this MediaWiki install via pingback

Add option for sharing info about this MediaWiki install via pingback

When $wgPingback is true, MediaWiki will periodically ping
https://www.mediawiki.org/beacon with basic information about the local
MediaWiki installation.  This data includes, for example, the type of system,
PHP version, and chosen database backend.

The pingback is sent via a deferred (post-send) update whenever $wgVersion
changes, using the updatelog table to ensure we don't send duplicate pingbacks.
A database lock ensures only one thread attempts to send the pingback, and a
cache key throttles attempts to no more than once per hour.

$wgPingback is false by default. The web installer has a checkbox for
controlling this option, and it is checked by default. This nudges new installs
to turn on pingbacks, but does not sneak this decision past sysops of existing
installs.

Change-Id: Ie43a6b46a07ebd9ccc1b9c3001f2ea02435d826b