MM-34758 Collapsed Reply Threads without mobile support (#17424)

Summary
added support for legacy clients accessing server
added collapsed_threads_supported param to viewChannel API and setPostUnread API

Ticket Link
https://mattermost.atlassian.net/browse/MM-34758

Related Webapp PR
mattermost/mattermost-webapp#7933

api4/channel.go

@@ -1330,7 +1330,7 @@ func viewChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	times, err := c.App.ViewChannel(view, c.Params.UserId, c.AppContext.Session().Id)
+	times, err := c.App.ViewChannel(view, c.Params.UserId, c.AppContext.Session().Id, view.CollapsedThreadsSupported)
 	if err != nil {
 		c.Err = err
 		return