Don't publish non-case-study unpublishings.

Currently we are sending placeholders rather than content-items to the publishing api for anything other than case studies. These don't register routes with the router. However, we are sending unpublishings for all unpublished editions, of whatever types, and these register redirects with the router. Therefore a non-case-study that has been unpublished and republished remains redirected.

To fix this, ensure that only unpublishings for case studies are sent to the content store.

A separate PR on content store will remove the already-sent unpublishings and unregister the redirects.

https://trello.com/c/FW0Ly34Y/99-bug-republishing-an-unpublished-document-still-redirects