Clean up virtual "_ft_rank" column for query construction of Ferret objects

Summary:
Ref T12974. Ferret object queries SELECT a virtual "_ft_rank" column for relevance ordering.

Currently, they always SELECT this column. That's fine and doesn't hurt anything, but makes developing and debugging things kind of a pain since every query has this , blah blah _ft_rank junk.

Instead, construct this column only if we're actually going to use it.

Mostly, this cleans up DarkConsole / query logs a bit.

Test Plan:
Viewed normal query results on various pages, viewed global search results, ordered Maniphest tasks by normal stuff and by "Relevance".

Viewed DarkConsole, saw no more "_ft_rank" junk on normal pages.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12974

Differential Revision: https://secure.phabricator.com/D18728