Video support #14
Description
WordPress 4.7 added support for custom video headers. This should be rolled into the Unique Headers plugin.
Branch video-support demonstrates how to control the video URL.
This does not work on inner pages, but only the front page. This is because WordPress 4.7 does not allow video headers on inner pages. It may be possible to override this behaviour, but I haven't figured out how yet.
It will be necessary to check if video support is enabled, and if so, show an extra meta box. We perhaps shouldn't use the existing meta box, because we need to show a fallback image. Alternatively, we could fallback to the default site wide header image, but this won't show on mobile, which will likely annoy many people.
Perhaps should leave a message alerting user that they should upload a fallback image if they've already selected a video.