calendar/CalendarEventPopover: fix bug where editing an event but the…

…n closing the modal could result in the event changing anyways

pass the event modal a copy of the event object so that it can't mutate the original

app/calendar/CalendarEventPopover.jsx

@@ -14,7 +14,7 @@ import HomeworkName from "ui/HomeworkName.jsx";
 
 export default class CalendarEventPopover extends Component {
 	edit() {
-		var item = this.props.item;
+		const item = Object.assign({}, this.props.item);
 		item.type = this.props.type;
 		this.props.openModal("calendarEvent", item);
 	}