Add support for header videos

[westonruter]

In Twenty Seventeen there is a header video feature: https://make.wordpress.org/core/2016/11/26/video-headers-in-4-7/

This is not working in Twenty Seventeen when the amp theme support is added. It is just shown as blank (as with header image in the theme):

[kienstra]

Request To Work On Issue

Hi @DavidCramer,
Would you mind if I worked on this?

[DavidCramer]

@kienstra - go for it!

[kienstra]

Steps To Test

Hi @csossi,
Could you please test this?

1. Visit this staging site. Your login details will be the same as other AMP staging sites like this, but feel free to Slack me if they don't work.
2. It currently has a YouTube URL as the header video, but there's a header image on mobile displays
3. Ensure that display well, without obvious styling issues
4. In the Customizer, click "Select video" to choose a different video:

5. Ensure that the video displays well on the front-end, and that it doesn't display on mobile devices

[csossi]

verified in QA