[Redesign] Clean up Slowvote

Summary: Ref T8099, A cleaner, more standard UI for Slowvotes. Because Slowvote is important.

Test Plan: Test a number of poll layouts and embeds.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13445