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)
Ah. I assume this is why we converted the above to class methods then. Just interrogate the class for its metadata keys rather than having to create an instance. Valid.