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:

Example transaction values in DB:

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T12623

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