Add Mark as Helpful to PonderAnswer

Summary: Ref T6920, this adds a basic controller for marking an answer as helpful and removes the negative voting. Any current positive vote is kept as helpful. New UI is needed here, but there is a separate task for redesigning Ponder overall.

Test Plan: Mark an answer as helpful, see count go up, remove helpful, see count go down. Test endpoint manually.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T6920

Differential Revision: https://secure.phabricator.com/D13834