Add episode duration to the DB

[wesbos]

Episode duration isn't something we have saved. IT would need to be calculated at import.

I initially thought we could do this when we get the transcripts, because we have ffmpeg out. BUT I think we can do it without ffmpeg?

Perhaps though the metadata of the mp3?

https://github.com/transitive-bullshit/get-mp3-duration https://stackoverflow.com/questions/50503460/get-duration-of-remote-mp3-file-without-full-download