Keystone v3 improvements.

Managers for project group membership, project user membership, project combined membership (users and groups); for group project membership, group users, group roles; for user project membership, user roles.
Generally, that's when you switch to something like Bluebird which linearizes your nested callbacks into a nice, understandable promise chain: https://github.com/maseb/thicket/blob/6f506c693dde8731ef7c3698240afd713c30decf/test/common/grove/storage/test-caching-data-store.js#L74-L100