events: Fill ics calendar with TZ-aware datetimes

We filled the ics calendar used to produce the ics output with naive
datetime specifications without a tiemzone. The ics calendar assument
UTC and so the times of the exprted ICS calender were off.

The database doesn't containt datetimes, but seperate dates and times
which also don't have timezone information. The times are assumend to be
the timezone of the server (TIME_ZONE setting in
config/settings/base.py).

Use astimezone() from pytz library to add time zone information to
datetime values handed to the ICS calendar.

Acked-By: schrader@molgen.mpg.de

mpicms/events/models.py

@@ -58,14 +58,14 @@ class Event(BodyMixin, BasePage):
     @property
     def start(self):
         if self.start_time:
-            return datetime.combine(self.start_date, self.start_time)
+            return datetime.combine(self.start_date, self.start_time).astimezone()
         return self.start_date
 
     @property
     def end(self):
         if self.end_time:
             date = self.end_date or self.start_date
-            return datetime.combine(date, self.end_time)
+            return datetime.combine(date, self.end_time).astimezone()
         return self.end_date
 
     def get_dict(self, request=None):