Upgrade message store to allow messages to be stored in multiple batches.

Currently inbound and outbound message models look like:

```python
class OutboundMessage(Model):
    # key is message_id
    msg = VumiMessage(TransportUserMessage)
    batch = ForeignKey(Batch, null=True)

class InboundMessage(Model):
    # key is message_id
    msg = VumiMessage(TransportUserMessage)
    batch = ForeignKey(Batch, null=True)
```

In order to support the new `batch-per-{channel, router, conversation}` model in Vumi Go these `ForeignKey` fields need to become `ManyToMany` fields.