Information about invitable roles in `/roles` endpoint

> /sites/$site/roles endpoint doesn't contain information about invites. A user can be invited as a viewer/follower, but these roles do not exist in that endpoint. I think we should either have a separate endpoint for invitation roles, or we should add all roles and mark them as selectable, invitable etc. We currently have a workaround for this, just like in Calypso, we manually add the viewer/follower roles depending on the privacy of the site, but I think this information should be coming from the API.

p7rcWF-Ap-p2