Update Slowvote voting methods to use sensible string constants

Summary: Ref T13682. Use API-friendly string constants instead of opaque integers in Slowvote voting methods.

Test Plan: Created, edited, and voted in polls with various voting methods. Examined database after migrations.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13682

Differential Revision: https://secure.phabricator.com/D21846