Add a music vertical

[gauss0818]

Add music data model and basic unit test for MusicContainerResource.

[xokker]

Thank you for your contribution!

I'm afraid I'm lacking the context for these changes, so can't do a proper review. But I'd like to highlight we already have CreativeWork, MusicPlaylist, MusicGroup, and MusicRecording models in the repo. We definitely wouldn't want to duplicate these classes.

[gauss0818]

@seehamrun said the existing playlist model was written as a dummy example and proof of concept. There is no one actually using it. There are only 2 implementation of PlaylistExporter/Importer (spotify and deezer), which are implemented as a proof of concept. We can write convertors and/or deprecate the playlist vertical.

Apple Music and YouTube Music is working on a new music data model which will contain playlists, tracks and releases. It would be better to name the directory as music instead of playlist.

[seehamrun]

we already have CreativeWork, MusicPlaylist, MusicGroup, and MusicRecording models in the repo. We definitely wouldn't want to duplicate these classes.

We actually already have 2 versions of CreativeWork, one in types/common/models/ and one in types/common/models.playlists and the one in playlists appears to be unused and should be deleted. @gauss0818 has already removed the copy of CreativeWork that was in this PR and used the one in types/common/models

For the remaining ones, the changes required for the new music vertical are not a 1:1 drop in replacement for these classes so its not simple enough to just rename that directory and add the missing fields. For example, in the existing model, tracks are included in the MusicPlaylist model but in this proposal they are required to be represented outside of the model. @gauss0818 do you have an external design document that contains the model proposals we can link here?

[gauss0818]

@seehamrun Here is the external DTP music data model design doc. Please reach out to me [email protected] if you don't have the commenter access.