The iTunes Festival channel has reappeared on Apple TV, not long before the festival kicks off in London. Apple’s iTunes Festival starts September 1 and runs through the whole month. As has been the case previously, the channel will let users watch live HD streaming video of the performances, as well as on-demand recordings after performances have ended. New acts, including Mary J. Blige, deadmau5, and Ed Sheeran, were recently added to the…