Add buttons to delete or free tasks from the queue

Summary: See T709. I also ran into a case in Drydock where this is useful for
testing/development.

Test Plan: Freed lease of a task; deleted a task.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T709

Differential Revision: https://secure.phabricator.com/D1469