Replace custom search endpoint with WP 5.0 search endpoint

[mharis]

I have not explored the new search endpoint that comes with WordPress 5.0 but we can probably replace our custom endpoint with the core search endpoint.

https://make.wordpress.org/core/2018/12/06/the-rest-api-in-wordpress-5-0/

[davebonds]

I definitely think we should explore replacing this with the core endpoint due to weirdness with our custom endpoint when no results are found.