Reduce the amount of ytdl requests done

[meew0]

Currently, two ytdl requests are required to play a video - one when queueing it and one when playing it back. This can be reduced to one by caching the entire video info downloaded and then using ytdl downloadFromInfo to play it. This could also be applied to saved videos, though here we have to make sure that the lethe-data.json file doesn't get too large.