Clean up Slowvote transactions a little

Summary: Ref T12685. Moves to xaction folder and sets description changes in transaction stories.

Test Plan: Make a poll, edit the description.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12685

Differential Revision: https://secure.phabricator.com/D17849