This reverts commit 7b0db234.

This reverts commit b3d8d6a4.

This adds support for reading metadata from Opus files. The metadata config
is copied from OggVorbis with just the 'track' field name changed.

Also make sure get_audio_file_data() in music/utils.py doesn't crash if no
bitrate information is set, since mutagen doesn't add a bitrate to Opus
info objects.

Fixes #244.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

If you ever need an empty flac file with metadata again:

1. Get a flac file (like https://archive.org/download/NineInchNailsTheSlip24bit96khz/01999999.flac)
2. Tag it with Musicbrainz Picard
3. Truncate it, keeping only tags with `ffmpeg -i in.flac -ss 0 -to 0.001 out.flac`

Thanks @HgO for the trick!