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