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