Allow disabled users to be typeaheaded in Differential

Summary: Fixes T3773. By default, the `/users/` datasource excludes disabled users (since it doesn't make sense to assign them tasks or make them reviewers, for example). However, for ApplicationSearch it does make sense to look for objects, e.g., authored by a disabled user.

Test Plan: Searched for disabled users in Differential.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3773

Differential Revision: https://secure.phabricator.com/D6834