search: Refactor full-text search

This patch adds a few changes. First, the full-text search query is
refactored so that MySQL can focus first on finding matching records in
its full-text search index (in the %_search table) in a subquery. Then
it can focus on matching records against tickets in an outer query
(whether ticket data, thread entries, user info, or organization inf was
matched).  This dramatically speeds up the query for large datasets by
about 20x (about three seconds instead of sixty-something).

Secondly, results are now sorted based on relevance, so the best hits
sort a the top of the list. This is accomplished by adding the
`relevance` to the query sort list via the extra() method. Then, since
no GROUP BY clause is really necessary, it is not added to the query,
which will allow the results not to be re-sorted by the ticket_id. (That
is, the remain sorted by the search relevance).

Third, the relevance has a lower limit of 0.3. Good hits seem to have a
much larger value (like hundreds), so setting a small value will help to
remove hits with barely any relevance to the search terms.