Remove "Create" option when user doesn't have the `create_guest_authors` cap

[johnbillion]

If the create_guest_authors capability is not granted to a user but they still have attribute_post_type for the given post type, the "Create" option still shows up at the bottom of the author attribution list even though the user cannot create a guest author.

The "Create" option should only appear when the user can create guest authors.

This does not affect vanilla use of the plugin, only when capabilities are customised.