calendar: Fix all day events

All time events are stored without a time in the database. The method
event.start returns a date instead of a datetime in that case. If so,
set the ics event to all_day.

mpicms/events/models.py

@@ -1,5 +1,5 @@
 import json
-from datetime import datetime
+from datetime import datetime, date
 from ics import Calendar, Event as ICSEvent
 
 from django.db import models
@@ -138,6 +138,8 @@ def ics(self):
                 url=event.full_url,
                 location=event.room
             )
+            if type(event.start) is date:
+                e.make_all_day()
             c.events.add(e)
         return '\n'.join(c)