Turn off spellcheck, etc, on main search input

Summary: Ref T12872, turns off all these "helpful" fields.

Test Plan: Type "phab" in main search and do not get a suggestion for "phablet".

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12872

Differential Revision: https://secure.phabricator.com/D18163