Convert PonderAnswer to modular transactions

Summary: Fixes T12624. Converts PonderAnswer over to modular transactions.

Test Plan:

• Add an answer
• Edit an answer
• Hide an answer
• Comment on an answer

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12624

Differential Revision: https://secure.phabricator.com/D17811