YT - attempt to dynamically decrypt sigs

I just opened this PR for discussion. The code is not currently functional as it relies on some methods which are (at present) forbidden by the Framework's RestrictedPython. In it's current form, calls to the YT URL Service generate a syntax error due to the use of Yield statements. Since the source code from the youtube-dl project relies heavily on the generator/yield functions, I don't believe it's feasible (or at least practical) to refactor the code in such a way as to avoid their use. If it's deemed reasonable to include that support in the framework, then there may yet be other hurdles to this implementation that I have as yet been unable to identify.

Ideally, if this can be made to work, it will drastically reduce the number of errors and associated error reports for YT vids in the channel and queue. We would no longer be playing the frustrating game of whack-a-mole where the static sig keys need to be updated on a daily basis.

Tagging for discussion: @jam, @sander1