Make Differential use viewer's token instead of viewer() token again

Summary: Ref T4100. This restores the simpler behavior. See discussion in T4100#107445

Test Plan: Used Differential search, saw my token.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4100

Differential Revision: https://secure.phabricator.com/D12447