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!