R1:c1887f0c1d55

Separate Slowvote poll status onto a dedicated object

Summary: Ref T13682. Prepares for use of API-friendly string constants rather than opaque integers.

Test Plan: Created and edited polls, opening and closing them. Grepped for affected methods.

Maniphest Tasks: T13682

Differential Revision: https://secure.phabricator.com/D21847