Allow per-site autoplay policies.

Denying autoplay can break websites that use it for stylistic effects. On the other hand, it can be increadibly annoying to be spammed with videos autoplaying.

By default, videos can autoplay so long as they have no audio (or the audio is muted). Alternatively, users can allow all videos to autoplay even with audio, or to deny all attempts to autoplay video. Users can make these decisions using the security pop-over on a per security-origin basis.