Migrate theme groups to an appropriate table

[Kyrch]

We currently have an inconsistency in the theme groups, so we decided to migrate to an appropriate table using the one-to-many relationship. We want to leave this until after the playlists have been released

• [ ] Create Anime Theme Group Model and update the Anime Theme Model
• [ ] Create db and elastic migrations
• [ ] Create new resource in Nova and change the field type on Anime Theme
• [ ] Create the events
• [ ] Create the policy and permissions
• [ ] Update the API
• [ ] Update API docs
• [ ] Create seeder
• [ ] Drop group column on anime theme table