Add check for correct content of MBA web page

The www.mba.ac.uk page will return a "200 OK" response even when PHP complains about not enough connections, so we need to test for actual content. From https://www.linuxjournal.com/content/bare-bones-monitoring-monit-and-rrdtool:

check host linuxjournal-website with address www.linuxjournal.com
    if failed
        port 443 protocol https
        with request / with content = "Become a Patron"
    then alert

@dble