Turn local_lsf_unification_cron into a scheduled task

[dlmsr]

This plugins runs the function insert_missing_helptable_entries once a day using the old cron functionality:

/**
 * Function to be run periodically according to the scheduled task.
 *
 * NOTE: Since 2.7.2 this function is run by scheduled task rather
 * than standard cron.
 * @package local_lsf_unification
 */
function local_lsf_unification_cron() {
    global $CFG, $pgdb;
    include_once(dirname(__FILE__) . '/class_pg_lite.php');
    include_once(dirname(__FILE__) . '/lib_features.php');

    $pgdb = new pg_lite();
    $connected = $pgdb->connect();
    $recourceid = $pgdb->connection;

    mtrace(
        '! = unknown category found, ? = unknown linkage found;' . 'Verbindung: ' .
        ($connected ? 'ja' : 'nein') . ' (' . $recourceid . ')'
    );

    insert_missing_helptable_entries(true, false);

    $pgdb->dispose();
}

This should be transformed into a scheduled task.