Update Slowvote poll status to use sensible string constants
Summary: Ref T13682. This prepares for modernizing Slowvote and exposing a more usable API.
Test Plan: Ran migrations, opened and closed polls.
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Maniphest Tasks: T13682
Differential Revision: https://secure.phabricator.com/D21848