Themes: Remove front page restriction from video header functions.
Adds a callback for determining when video headers should be displayed in themes supporting custom headers. By default, video headers are only displayed on the front page of a site.
Theme authors may set a custom callback by passing 'video-active-callback' => 'mytheme_video_active_callback' as an argument. The default callback is is_front_page().
This introduces the new function is_header_video_active() - returns true on pages that should display video headers. The return value can be filtered using the new filter of the same name.
Props flixos90, bradyvercher, peterwilsoncc, joemcgill.
Fixes #38738.