Enable site admins to create guest authors

[rmccue]

In a multisite context, site admins do not have the ability to create guest authors, as only super-admins can create users. This creates a painful experience where site admins become a bottleneck for editorial workflows.

The key reason WordPress does not allow site admins to create users is since common attributes like name, avatar, etc are shared across sites. However, with guest authors, each guest author is likely to be unique to the editorial context, and so uniqueness may be desirable. Additionally, there is no real downside to having duplicate users across sites, as the user will never log in and hence SSO does not matter.

To match the single-site design, site admins should be able to create new guest authors directly.