Video: Add Download Handout

Ticket: [BLD-1000](https://edx-wiki.atlassian.net/browse/BLD-1000)

Provide an upload button for authors to provide students with an option to download a handout associated with a video (of arbitrary file format).

**Import/Export:**
Added new node `<handout />`

```xml
<video>
    <handout src="http://www.example.com/handout"/>
</video>
````

@auraz , @jmclaus , @valera-rozuvan, @srpearce  please review.