Investigate whether there is an issue with timezone of Event.end

Ghost User
2017-08-04T21:52:22+02:00

When testing !506, I got the following warning. We should look into this.

```
root@ivo:/etc/dockerts/beta# ./manage.py.sh sendmembershipnotification --dry-run
/usr/local/lib/python3.5/site-packages/django/db/models/fields/__init__.py:1451: RuntimeWarning: DateTimeField Event.end received a naive datetime (2017-06-28 21:21:28.118351) while time zone support is active.
RuntimeWarning)
```