Add back the functions that added person tags on post save

Addresses https://github.com/GigaOM/gigaom/issues/6338
This function was removed from this file on purpose. It was relocated to here: https://github.com/Camwyn/go-quotes/blob/6338-person-tags-again/components/class-go-quotes-admin.php#L141

I'm guessing that [this line](https://github.com/Camwyn/go-quotes/blob/6338-person-tags-again/components/class-go-quotes-admin.php#L184) should be tweaked to read: `do_shortcode( $post->post_content );` rather than `do_shortcode( $content );`?