Added new mode to remove ticket meta data of invalid users.
Added new mode to remove ticket meta data of invalid users.

otrs.CleanupTicketMetadata.pl --invalid-users

    # Deletes ticket/article seen flags and ticket watcher entries of users which have been
    #   invalid for more than a month. Run this to clean up this kind of data after invalidating
    #   significant amounts of users.

djoudi/otrs-gitimport-test,djoudi/otrs-gitimport-test,djoudi/otrs-gitimport-test