Slowvote - add ability to destroy a poll

Summary: Fixes T5773.

Test Plan: Made a poll and voted on it. Deleted it via ./bin/remove destory V1. No errors and the poll is gone.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5773

Differential Revision: https://secure.phabricator.com/D10167