Allow standard custom fields to be indexed in global fulltext search

Summary: Fixes T6399. This allows you to use global search to find projects by searching for text in their descriptions.

Test Plan: Added a unique word to a project description, reindexed it, searched, got a hit.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6399

Differential Revision: https://secure.phabricator.com/D10748