Set a default value on BooleanField (Log.valid)

To mute the following Django 1.6+ warning, set a default value `None` to BooleanField `Log.valid`:

```
trustedhtml.Log.valid: (1_6.W002) BooleanField does not have a default value.
        HINT: Django 1.6 changed the default value of BooleanField from False to
 None. See https://docs.djangoproject.com/en/1.6/ref/models/fields/#booleanfield
 for more information.
```