convert a keyword search into a faceted term search

* provided we can convert the search keyword into an authoritative facet term
* if more than one facet is matched, we'll use the first one with the highest priority configured

related to these pull request: https://github.com/GigaOM/gigaom/pull/5114 and https://github.com/misterbisson/scriblio-authority/pull/76

see https://github.com/GigaOM/legacy-pro/issues/3995
the tradeoff is that if we explicitly pass in the parameters (which would be `$this->facets->selected_facets` i think), then code from other classes can call this function too.