events: Remove extra "\n" from ics output

We don't need to use "\n" joiners for the output of the ics.Calendar
iterator, because the lines already contain "\r\n" termination.

If fact, ics.Calendar.__str__() returns the calendar as an ics formated
string, so just return the icx calendar itself.

Newer versions of ics have better serialisation, but we don't want to
complicate things by library updates now.

mpicms/events/models.py

@@ -136,7 +136,7 @@ def ics(self):
             if type(event.start) is date:
                 e.make_all_day()
             c.events.add(e)
-        return '\n'.join(c)
+        return c
 
 
     class Meta:  # noqa