Feature/my queue

Can give me suggestion of my solution in the follow:

1. in line 11 of `QueueItemsController` class, because I am using `validates_uniqueness_of :video_id, scope: :user_id` in the `QueueItem` class to restrict video can't add two times to the user's my queue, so I use save method to check if it invalidate, then set flash.

2. in line 19 of `QueueItemsController` class, I use `find_by` to find the queue item in the current user's queue, because `ActiveRecord#find` will raise exception, when the queue item is not own by current user, or not exist.

Thanks.