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