Add user type Manager to Authors setup

Adjustment for #1692 

This allows a user type `Manager` to be selected when creating Authors. This Manager user type is allowed to access the Authors section to manage Authors with a user type of Manager or Author. Developers are hidden from this user type for obvious reasons.

The Authors section alterations are the only difference between a simple Author type and Manager, so all other System and Blueprint pages are accessed in exactly the same way as Authors.

This is a very small change, for some really helpful and useful additional functionality for clients who expect to be able to manage website Authors.