Support cover art in COVERART field in addition to METADATA_BLOCK_PICTURE field for ogg files

Right now, api/funkwhale_api/music/metadata.py looks for picture data in ogg files only in the metadata_block_picture section. While unofficial and depricated, some old ogg files store their cover art in the coverart section instead. See https://wiki.xiph.org/VorbisComment#Unofficial_COVERART_field_(deprecated)

It would be great if funkwhale could still use these images without having to mass convert old ogg files.