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This adds the ability for themes to add support for videos in custom headers by passing `'video' => true` as an argument when adding theme support for custom headers. Custom video headers are managed through the “Header Visuals” (i.e. “Header Image”) panel in the Customizer where you can select a video from the media library or set a URL to an external video (YouTube for now) for use in custom headers. This introduces several new functions: `has_header_video()` – Check whether a header video is set or not. `get_header_video_url()` – Retrieve header video URL for custom header. `the_header_video_url()` – Display header video URL. `get_header_video_settings()` – Retrieve header video settings. `has_custom_header()` – Check whether a custom header is set or not. `get_custom_header_markup()` – Retrieve the markup for a custom header. `the_custom_header_markup()` – Print the markup for a custom header. And a new file, `wp-includes/js/wp-custom-header.js` that handles loading videos in custom headers. This also enables video headers in the Twenty Seventeen and Twenty Fourteen themes. Props davidakennedy, celloexpressions, bradyvercher, laurelfulford, joemcgill. Fixes #38172. git-svn-id: https://develop.svn.wordpress.org/trunk@38985 602fd350-edb4-49c9-b593-d223f7449a82