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
`$this->facets` is the `Facets` object that contains this `Facet_Searchword` instance. it's passed in and set [in `Facet_Searchword`'s constructor](https://github.com/misterbisson/scriblio/blob/7c843df82ccca560c1a8fa641ee7e809d5f959b8/plugin/class-facet-searchword.php#L13).