Unlearn replying to thread_broadcast messages with its own broadcast

The Go Slack client is missing the field on the MessageEvent that would allow
us to identifier who the sender of the message was. Specifically, the `root` key
as shown here: https://api.slack.com/events/message/thread_broadcast

Until that can be done, this disables the bot's ability to reply to broadcasted
messages with a broadcast.

handler/message_actions.go

@@ -126,6 +126,17 @@ func shouldDiscard(m *slackevents.MessageEvent) (string, bool) {
 		return fmt.Sprintf("message has subtype %s", m.SubType), true
 	}
 
+	// TODO(theckman): as of now the bot is unable to recognize whether a
+	// thread_broadcast message was from itself or not. We need to add support
+	// for parsing the root subkey of the thread_broadcast message event:
+	// https://api.slack.com/events/message/thread_broadcast, because that's the
+	// user we care about.
+	//
+	// Once that's added we should discard thread_broadcast messages from
+	// ourselves.
+	//
+	// Also, see related TODO in responder.go.
+
 	tss := strings.Split(m.TimeStamp, ".")[0]
 
 	// we assume this is a well-formed message

handler/responder.go

@@ -152,9 +152,14 @@ func (r response) respond(ctx context.Context, mentionUser, useMentions, ephemer
 
 	// if it's a command that was triggered in a shared thread reply
 	// we should share our reply with the channel too
-	if len(subType) > 0 && subType == "thread_broadcast" {
-		opts = append(opts, slack.MsgOptionBroadcast())
-	}
+	//
+	// TODO(theckman): re-enable this functionality once gopher is able to
+	// recognize thread_broadcast messages from itself. See TODO in
+	// message_actions.go for more context.
+	//
+	// if len(subType) > 0 && subType == "thread_broadcast" {
+	// 	opts = append(opts, slack.MsgOptionBroadcast())
+	// }
 
 	if len(attachments) > 0 {
 		opts = append(opts, slack.MsgOptionAttachments(attachments...))