Replace getID3 with ffprobe

[markspolakovs]

We currently use getID3 to extract metadata and bitrate from uploaded songs, which can only handle certain types of files -which is where the file type restriction comes from.

Switch to using ffmpeg/ffprobe, which can in theory handle any file type.

This would not be adding a dependency, as we already have ffmpeg to handle the transcoding - in fact it would be removing one.