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)
Honestly I prefer the previous invocation of instance methods rather than a class method here. What benefit does this provide over instance methods?