Guest author role

[johnbillion]

• Create new user role of "Guest Author" with no permissions
• This allows a guest author to be represented by an actual WordPress user with a role of guest author. This allows author archives, profiles, avatars, social links, opengraph, etc to work without any customisation, and allows for user promotion at a later date. See #2 for more info.
• Fine grained permission for other users to manage guest authors to come later