Channels [models, API]

See #170 (closed):

• Model/database layer to represent channels: a channel a database entry linking to:
  • An artist
  • An actor (to broadcast activities on federation)
  • An owner (the account who owns the channel, expressed in the attributed_to field)
  • A library (to store the channel uploads)
• CRUD API to manage channels
• Django admin module
• Feature flag to disable channels
• Exclude channels-related tracks, albums and artists by default on existing endpoints to preserve backward compatibility

Ready for review @funkwhale/reviewers-python