Offer individual audio tracks for download

[JulianKniephoff]

#1079 added the ability to share display audio. This made it possible for the resulting videos to have two potential audio tracks: the mic, and the display audio. Right now, we just mix those together and use the result as single audio track for both videos.

To offer the user a bit more flexibility, we might want to offer the individual audio tracks as downloads, alongside the videos, so that users can do their own postprocessing.