Fix naive datetime runtime error

Ghost User
2019-05-30T12:25:15+02:00

### Short description

When running the `minimiseregistrations` command, the following error is thrown:
```
/usr/local/lib/python3.7/site-packages/django/db/models/fields/__init__.py:1363: RuntimeWarning: DateTimeField Entry.updated_at received a naive datetime (2019-04-25 00:00:00) while time zone support is active.
RuntimeWarning)
```

Using a `timezone` object instead of a `datetime` object fixes this.