REST API endpoint for guest authors

[johnbillion]

Introduce a full CRUD endpoint in the REST API for guest authors.

This is needed because we're going to allow lower level users (eg. Authors) to create guest authors, and I don't want to mess about with the permissions of the /users endpoint, and using a new endpoint abstracts this away from user storage.

Mostly expects and exposes the same fields as the /users endpoint, except:

• Email address should be optional, need to decide on strategy for populating users with a fake email address if one isn't provided, or whether allowing no email address works well
• User login should be optional and will be generated if one isn't provided
• role won't be required as the user will always have the guest author role
• The capabilities required to read and write to this endpoint need to be filterable
• Need to consider whether this endpoint will behave the same as the /users endpoint for non-authenticated requests, eg. by listing all guest authors that have published content