Themes: Enable video in custom headers.

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.
Built from https://develop.svn.wordpress.org/trunk@38985


git-svn-id: http://core.svn.wordpress.org/trunk@38928 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.7-alpha-38984';
+$wp_version = '4.7-alpha-38985';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.