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
This improves UX, since an image added will be displayed on front end as opposed to not at all. This fix added the front page's featured image above the front page content, similar to how it's handled in the other panels. Also it removed code that was setting the front page's featured image as a fallback to the custom header, and updated the conditions that add the `has-header-image` to remove reference to the front page's featured image.
Props laurelfulford.
Fixes#38402.
Built from https://develop.svn.wordpress.org/trunk@38868
git-svn-id: http://core.svn.wordpress.org/trunk@38811 1a063a9b-81f0-0310-95a4-ce76da25c4cd
