monitoring: monitor mediawiki jobs

As a consequence of the last jobrunners outage, we add checks on the job
queue length both for queued and running jobs. With these checks, we
would have been warned of the problem way earlier.

Change-Id: I054e11cea019369813eb92e2c62e7e10c5ac47cb
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>