Allow structured destruction of Maniphest tasks

Summary:
Ref T4749. Ref T3265. Ref T4909.

• Support ManiphestTasks in bin/remove.
• Transactions need manual help for now.

Test Plan:

• Destroyed tasks.
• Examined log to verify task, transaction, and comment destruction.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3265, T4749, T4909

Differential Revision: https://secure.phabricator.com/D8944