Refactor full-text search on several tables (comments, dmails,
forum_posts, forum_topics, notes, and wiki_pages) to use to_tsvector
expression indexes instead of dedicated tsvector columns. This way
full-text search works the same way across all tables.
API changes:
* Changed /wiki_pages.json?search[body_matches] to match against only
the body. Before `body_matches` matched against both the title and the body.
* Added /wiki_pages.json?search[title_or_body_matches] to match against
both the title and the body.
* Fixed /dmails.json?search[message_matches] to match against both the
title and body when doing a wildcard search. Before a wildcard search
only matched against the body.
* Added /dmails.json?search[body_matches] to match against only the dmail body.
