Order search results by relevance, rather than by date.

The ordering logic is as follows:
 * Full sentence matches in post titles.
 * All search terms in post titles.
 * Any search terms in post titles.
 * Full sentence matches in post content.

Each section and any remaining posts are then sorted by date.

Introduces some filters:
 * wp_search_stopwords, to filter stop words ignored in WHERE.
 * posts_search_orderby, to filter the ORDER BY when ordering search results.

props azaozz, wonderboymusic.
fixes #7394.


git-svn-id: https://develop.svn.wordpress.org/trunk@25632 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -3702,19 +3702,6 @@ function get_file_data( $file, $default_headers, $context = '' ) {
 	return $all_headers;
 }
 
-/**
- * Used internally to tidy up the search terms.
- *
- * @access private
- * @since 2.9.0
- *
- * @param string $t
- * @return string
- */
-function _search_terms_tidy($t) {
-	return trim($t, "\"'\n\r ");
-}
-
 /**
  * Returns true.
  *