R1:9dad49472c04

Move Slowvote vote types to a separate object

Summary: Ref T13682. Extract Slowvote vote types to a separate object, to prepare for turning them into API-friendly string constants.

Test Plan: Created, edited, and voted in Slowvote polls. Grepped for affected constants.

Maniphest Tasks: T13682

Differential Revision: https://secure.phabricator.com/D21845