Hint at thread replies when messages are unthreaded (slack) (#684)

bridge/slack/helpers.go

@@ -254,6 +254,13 @@ func (b *Bslack) populateReceivedMessage(ev *slack.MessageEvent) (*config.Messag
 		}
 	}
 
+	// For edits, only submessage has thread ts.
+	// Ensures edits to threaded messages maintain their prefix hint on the
+	// unthreaded end.
+	if ev.SubMessage != nil {
+		rmsg.ParentID = ev.SubMessage.ThreadTimestamp
+	}
+
 	if err = b.populateMessageWithUserInfo(ev, rmsg); err != nil {
 		return nil, err
 	}

bridge/slack/slack.go

@@ -293,6 +293,12 @@ func (b *Bslack) sendRTM(msg config.Message) (string, error) {
 		return "", err
 	}
 
+	// Handle prefix hint for unthreaded messages.
+	if msg.ParentID == "msg-parent-not-found" {
+		msg.ParentID = ""
+		msg.Text = fmt.Sprintf("[thread]: %s", msg.Text)
+	}
+
 	// Handle message deletions.
 	if handled, err = b.deleteMessage(&msg, channelInfo); handled {
 		return msg.ID, err