Polymorphic model and migration

[Cedev]

As a masochistic tree-concern user, I'd like to be able to make Trees of polymorphic models.

• [ ] Make sure Tree work with a polymorphic relationship
• [ ] Add a migration helper to support a polymorphic relationship
• [ ] Upgrade to a polymorphic relationship from the monomorphic one

[Cedev]

:-1: Polymorphism in rails is terrible.