R1:07fdcde87eaf

Provide viewer to CustomFields in ApplicationSearch

Summary: Fixes T4663.

Test Plan:
Added a custom field which accesses the viewer's username.

{F133249}

Reviewers: carl, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4663

Differential Revision: https://secure.phabricator.com/D8595