Use Rummager as search backend

Replacing soon to be decommissioned Finder API which is still in use for
sharing schemas.

[Trello ticket](https://trello.com/c/5Xlpt8Mn/306-make-finder-frontend-use-rummager-for-results-3)
I'm not sure I agree with the logic in a60bde8 for renaming this from slug to link. We refer to the "url route" to a thing as a slug on `Artefact` and in many other places, so I think I'd prefer to keep the previous naming for clarities sake.