Implement faux fullscreen when `allowfullscreen` is not enabled on an embed

[joepie91]

If allowfullscreen is disabled, the embedded viewer should fall back to navigating the parent frame to the embed URL when the fullscreen button is clicked, to create a 'faux' fullscreen mode. This allows for embedding on sites/services where arbitrary HTML input is disallowed, or where enabling allowfullscreen is undesirable for technical purposes.

Ideally, this would work for the 'presentation mode' button as well; that would, however, involve an extra flag in the URL to automatically enable presentation mode after navigating.