Valera/fix multiple videos on page

**Description**

If there are several videos on page, make sure that the global onYouTubeIframeAPIReady function is properly setup so that all callbacks are trigerred when the YouTube API loads.

**Jira issue**

[BLD-972](https://edx-wiki.atlassian.net/browse/BLD-972)

**Reviewers**

- @rocha