R1:d53187e10a4d

Make "Create Subtask" work properly in EditEngine

Summary: Ref T9908. This fixes "Create Subtask" so it works with the new stuff. Mostly straightforward.

Test Plan: Created some subtasks.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9908

Differential Revision: https://secure.phabricator.com/D14706