Change ics calls to documented API

Fixes #412

Author: hroncok

naucse/views.py

@@ -834,7 +834,7 @@ def course_calendar(course):
 
 
 def generate_calendar_ics(course):
-    calendar = ics.Calendar()
+    events = []
     for session in course.sessions.values():
         if session.start_time:
             start_time = session.start_time
@@ -851,9 +851,9 @@ def generate_calendar_ics(course):
                         session=session.slug,
                         _external=True),
         )
-        calendar.events.append(cal_event)
+        events.append(cal_event)
 
-    return calendar
+    return ics.Calendar(events=events)
 
 
 @app.route('/<course:course>/calendar.ics')