Use `sort`, not `text`, for search index table

Summary: Ref T1191. The index's case sensitivity depends on the column type. Using `text` makes the search case-sensitive, which is not desirable.

Test Plan: After adjustment, searched for "PROJECTS" and found hits against "projects".

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10619