Added a Consumer user type

My motivation for adding this user type is for API clients that will inevitably expose the authentication token during requests. I wanted a user type that would not be able to do any damage to the site or data if someone were to 'steal' an authentication token, therefore this user type only has Read access.

Maybe someone more familiar with the various parts of the system (content entry, content asset, snippet, theme asset, etc.) can tell me if I've enabled something not required or disabled something that should be required.