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.
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D18514