Add foreign keys where necessary

[simonv3]

These weren't added in the initial migration but they should be there. Every model that has a relationship to another query should have foreign keys. We should add migrations for this. High use models should probably take priority.

Priority:

• track_groups
• track_group_items
• tracks
• playlists
• playlist_items
• user_groups
• users
• plays

Lower priority:

• credits
• user_membership
• etc