[mailman] Use xapian as a backend for fulltext search
    
    The whoosh engine doesn't seem to be maintained anymore and when generating the
    index it got stuck when the size was about 20 GB. Xapian on the other hand looks
    more stable and much quicker when generating the index.
    
    Unfortunately the xapian-haystack package is not available in Fedora/EPEL yet
    and waiting for review to be finished.
    
    Link to review bug: https://bugzilla.redhat.com/show_bug.cgi?id=2313507
    
    Signed-off-by: Michal Konecny <mkonecny@redhat.com>