Create a virtual "core" field in the Ferret engine for "title and body together"

Summary: See PHI46. The core: function means "find results in either the title or body, but not other auxiliary fields like comments".

Test Plan: Searched for text present in the title (yes), body (yes), and comments (no) with the core:... prefix.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D18514