WIP: Begin work on "Multiple track artists"

Related issue: #646 (closed)

To-Do

• Write TrackCredit model and serializer
• Write migrations for TrackCredit
• Reimplement majority of funkwhale_api.music.metadata to account for
  • Tags with multiple fields and values
  • Tags with multiple list delimiters and preprocessors (i.e. feat. Artist -> TrackCredit(type=FEATURED, artist=Artist))
• Refactor code using class Metadata to be in line with changes
  • api/tests/music/test_{metadata,tasks}.py
  • api/funkwhale_api/music/{tasks,models}.py
• Test importer on decently big library
• Resolve linter errors

This Merge Request includes:

• Tests
  • Model, serializer
  • New tasks
  • Metadata scanner
• A changelog fragment (cf https://docs.funkwhale.audio/contributing.html#changelog-management)