MDL-37781 Check for switching the workshop phase without waiting for cron

MDL-37781 Check for switching the workshop phase without waiting for cron

Now we are checking for the need to switch the phase when the main
view.php is displayed to any user (previously this was done via cron
only). To give our subplugins (such as the scheduled allocation method)
a chance to do what they need to do, the workshop_viewed event is
triggered before the phase is actually switched.