MDL-36621 calendar: Ical event should display source info
MDL-36621 calendar: Display subscription info in calendar event popups
MDL-36621 calendar: cache calendar subscriptions and showicalsource setting

Now MUC is used to cache subscription information when the constructor for an event is called, which is child of a ical subscription.
A new setting calendar_showicalsource is introduced which controls if the ical source should be displayed along with events in the calendar
Also minor changes has been done in places to make sure subscription id is always associated with ical events