Support organizations

[marcua]

ayb's data model supports two types of entity --- user and organization. The rest of the implementation (e.g., registering, logging in, etc.) assumes a user is signing up. Some notes for when we want to support organizations

• How do you handle registration? Does the organization need an email address, or does a user create an organization and share administrative permissions with other users?
• How do you handle permissions? Can organizations issue keys, or is that left to users?
• How do you handle partially complete state? For example, a user that starts registering but doesn't verify an authentication method will leave behind an unverified Entity of type User. Should you allow the first verified registration to flip the type of an Entity? Answering this relies on some details of the first bullet.