Support thread_broadcast messages

[huyz]

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.