Allow changing of the Order.date_placed field

Removes the auto_now_add=True. Sets now as the default, but allows changing the date_placed field explicitly if an order is updated after it has been placed.

Fixes #1529
For customizability this structure allows implementations to change the date_placed logic:

```python
def save_date_placed(self):
    if self.date_placed is None:
        self.date_placed = now()

def save(self, *args, **kwargs):
    self.save_date_placed()
    return super(Parent, self).save(*args, **kwargs)
```