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>