Set dm_channel_id directly from the message object

Allows us to set the channel id correctly for non-DM messages,
such as mentions that create a thread with createThreadOnMention
where the initial message is forwarded to the thread.

src/data/Thread.js

@@ -310,6 +310,7 @@ class Thread {
       body: messageContent,
       is_anonymous: 0,
       dm_message_id: msg.id,
+      dm_channel_id: msg.channel.id,
       attachments,
       small_attachments: smallAttachments,
     });