Reimplement Slowvote transactions using modular transactions

Summary:
Fixes T12623. Adds new modular transactions to Slowvote. Also converts
the shuffle column to bool for consistency with other boolean-ish columns.

Test Plan:
Create a new vote, modified everything that could be modified from the web UI,
observed expected timeline.

Example timeline: {F4938843}

Example transaction values in DB: {F4938850}

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T12623

Differential Revision: https://secure.phabricator.com/D17830