closes https://github.com/TryGhost/Arch/issues/45
- This endpoint is here to keep the convention of being able to fetch the resource by it's slug through a `GET /{resource_name}/slug/:slug`. It has identical output as the `GET /collections/:id` endpoint
- The alternative would be having an alias and try fetching by :id and then by slug if the result for id was null, but that would be a completely new pattern we have not used anywhere else yet.
