Provide Maniphest auxiliary fields access to the viewer and task

Summary:
Maniphest auxiliary fields currently do not have access to the viewing user or task. This is fine for very simple fields, but insufficient for more complex fields. Generally, bring these in line with DifferentialFieldSpecifications.

This supercedes the additional $user/$viewer threading provided by D5247 and provides viewers to all fields, as well as access to the task object itself.

Test Plan: Created, viewed and edited a task with custom fields. Created a similar subtask.

Reviewers: hach-que, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2575

Differential Revision: https://secure.phabricator.com/D5280