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