Implement placeholders for status and preview API
Created by: eseliger
Adds the remaining bits for the API and fixes compilation of both backend and frontend. Don't look at frontend code, I just transformed it so much that is compiles, but not more.
For the backend, I spent some more effort and gathered back the previewRepositoryComparison resolver from master. Still outstanding is the handling for changesets where r.changeset
is nil
(Unpublished/publishing), and the resolvers with TODOs.