youtube.lua: descramble "n" URL parameter to fix throttling issue

YouTube playback is currently broken due to heavy data transfer throttling that was fully rolled out last week, see #26174 (closed) This is similar to the URL signature scrambling, and is solved here reusing the same proven design. Although the descrambling javascript for this is an order of magnitude more complex than for URL signatures, it's still fairly workable; considering the hindsight other analysts have provided since the apparition of the feature back in June, I'm fairly confident this solution will be as sustainable as the URL signature descrambling. The implementation still lacks more live testing over time, but I see no reason to withhold an initial release.

If approved, please merge and backport this to 3.0 without unnecessary delay, as people are looking forward to this fix to restore the feature.