PLT-4430 improve slow channel switching (#4331)

* PLT-4430 improve slow channel switching

* Update client side unit tests

* Convert getChannelsUnread to getMyChannelMembers and address other feedback

* Pull channel members on websocket reconnect

api/command_join.go

@@ -38,7 +38,7 @@ func (me *JoinProvider) DoCommand(c *Context, channelId string, message string)
 	} else {
 		channels := result.Data.(*model.ChannelList)
 
-		for _, v := range channels.Channels {
+		for _, v := range *channels {
 
 			if v.Name == message {