Modified manner in which an event was added to the EventListModel:
rather than adding the event before sending it to the database, the
AddEventRequestObserver waits for the server to set the ID, store the
event, and respond with the modified event. AddEventRequestObserver then
calls the new function in EventListModel, addEventFromObserver, which
makes sure that the ID of the event has been set and, if it has, adds it
to the EventListModel.