Add User Model

[Chris Brown]

I don't think this needs to be complicated - in fact it could be as simple as adding an email + ID (another UUID that doesn't change), but pretty quickly we're going to want a way to separate out predictions and models amongst users.

My proposal is the following:

• Add an OwnerID and email (for easier record keeping) to the tokens field
• Add an owner field to predictions + models
• Restrict CRUD actions to owners (except for the webhook)