Remove flags and tokens upon object destruction

Summary: When destroying an object, also remove all associated flags and tokens.

Test Plan: Awarded a Maniphest task with a token and flagged it as well. Destroyed it with ./bin/remove destroy and saw flag and token removed from the database.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13112