Simplify "Tasks" field in Differential

Summary:
Ref T11114. Keep UI, throw everything else away.

Includes an imperfect-but-not-too-awful fix to keep the field actually working.

Test Plan: Edited tasks from CLI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17088