R1:211a93529b36

Implement DestructibleInterface for dashboards and panels

Summary: Fixes T5471.

Test Plan: Used `bin/remove destroy` to destroy a dashboard and a panel.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5471

Differential Revision: https://secure.phabricator.com/D10283