R1:809ae8175259

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