Bring in mediawiki-vendor-integration

The job tests mediawiki/core and mediawiki/vendor together on patchset
submission.  It clones both repositories, attempting to match branch of
the proposed patchset then run the whole MediaWiki core PHPUnit test
suite.

The job has been experimented for a month or so (by using the
experimental pipeline which is triggered on 'check experimental').  I
believe it is good enough now to be triggered on every patches.

Make it non voting and will only trigger on master / REL branches for
now.

Experimental configuration was:
https://gerrit.wikimedia.org/r/#/c/141828/

Change-Id: I23164b7bbd7b1fb0309ec84a6d349858f06e3fe7
JJB: https://gerrit.wikimedia.org/r/#/c/141819/