[PLT-7362] Add post' root ID to APIv4 addChannelMember to render added user (as system post) at RHS (#7730)

* add post' root ID to apiv4 addChannelMember to render added user (as system post) at RHS

* add check to post_root_id parameter

* add AddChannelMemberWithRootId function for backward compatibility

api/channel.go

@@ -649,7 +649,7 @@ func addMember(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 
 	c.App.Go(func() {
-		c.App.PostAddToChannelMessage(oUser, nUser, channel)
+		c.App.PostAddToChannelMessage(oUser, nUser, channel, "")
 	})
 
 	c.App.UpdateChannelLastViewedAt([]string{id}, oUser.Id)