Twenty Seventeen: Better handling of custom headers when no image is set.

This ensures that a standard header is shown on the front page whenever
a header video is set without a header image if the video doesn't load,
e.g., on mobile sizes or if the JS doesn't fire.

This adds a new class, `.has-header-video` that is added whenever the
`wp-custom-header-video-loaded` event is fired, which is then used to style
the custom headers along with `.has-header-image` whenever a header image
is available. This also changes the class name on the custom header media
wrapping `div` from `.custom-header-image` to `.custom-header-media`.

Props laurelfulford, joemcgill.
Fixes #38995.
Built from https://develop.svn.wordpress.org/trunk@39413


git-svn-id: https://core.svn.wordpress.org/trunk@39353 1a063a9b-81f0-0310-95a4-ce76da25c4cd