[ticket/11040] PostgreSQL fulltext search improvement

PgSQL fulltext search queries doesn't look up both post_subject and post_text
http://tracker.phpbb.com/browse/PHPBB3-11040

https://github.com/phpbb/phpbb/pull/1054 and https://github.com/phpbb/phpbb/pull/1770 need to be merged before this PR and then tests will pass.

PHPBB3-11040