#558 Rewrite all success/error callbacks as Promises

[Anonymous]

Originally created by: MeoMix

Promises are way better for a multitude of reasons.

First off, all views which currently leverage callbacks need to check their isDestroyed property in the success/error event handlers. With promises the views would be able to detach their event listener when destroying themselves.

Furthermore, it would reduce the complexity of YouTubeV3API by providing a more consistent interface for responding to multiple, chained XHRs.