Make "text" custom fields appear in ApplicationSearch again

Summary:
Fixes T11675. This capability was erroneously (probably?) removed in D14766.

This search implementation (which uses exact match) probably isn't perfect for all cases of "text" fields, but empirically it seems to be what a significant number of users are after.

Test Plan:
Searched for a custom text field value.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11675

Differential Revision: https://secure.phabricator.com/D16582