Support `thread_broadcast` messages

Running `mmetl check slack`, I get
```
2020/08/14 21:22:25 Slack Import: Unable to import the message as its type is not supported. post_type=message post_subtype=thread_broadcast
```

These messages are the thread replies that are also sent to the main thread, which is the default display behavior of Mattermost, isn't it?  So it should be straightforward to import these messages, I imagine.