Add a one-click "Scuttle Task" button to Maniphest

Summary: Fixes T4657. See that task for discussion of edge cases.

Test Plan: {F132941}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: chad, carl, epriestley

Maniphest Tasks: T4657

Differential Revision: https://secure.phabricator.com/D8590