R1:e625c91867ec

Pass viewer to all ManiphestTaskQuery objects

Summary: Ref T603. Prepare for conversion to a policy-aware query.

Test Plan: Browsed various interfaces which use this stuff.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D6928