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)
Figured this out: https://github.com/alphagov/finder-frontend/pull/72/files#diff-5bc178ec5ca9e427578dd8a51c815730R66