MM-56071: Track multiple threads (#25775)

We can have 2 types of threads open at the same time. One from
the thread view, and another from RHS.

We add another variable to distinguish between the 2 states.

In future, if we have the ability for more than 2 threads, then
we would need to track by threadID.

https://mattermost.atlassian.net/browse/MM-56071

```release-note
NONE
```

server/channels/api4/websocket_test.go

@@ -441,7 +441,12 @@ func TestWebSocketPresence(t *testing.T) {
 	require.Nil(t, resp.Error)
 	require.Equal(t, resp.SeqReply, wsClient.Sequence-1, "bad sequence number")
 
-	wsClient.UpdateActiveThread("threadID")
+	wsClient.UpdateActiveThread(true, "threadID")
+	resp = <-wsClient.ResponseChannel
+	require.Nil(t, resp.Error)
+	require.Equal(t, resp.SeqReply, wsClient.Sequence-1, "bad sequence number")
+
+	wsClient.UpdateActiveThread(false, "threadID")
 	resp = <-wsClient.ResponseChannel
 	require.Nil(t, resp.Error)
 	require.Equal(t, resp.SeqReply, wsClient.Sequence-1, "bad sequence number")