Update Slowvote to use sensible string constants for response visibility

Summary: Ref T13682. Migrate and update Slowvote to use API-friendly string constants for response visibility, not opaque integers.

Test Plan: Created and edited slowvotes, changing response visibility.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13682

Differential Revision: https://secure.phabricator.com/D21844