Make adding an answer to a Ponder question a QuestionTransaction operation

Summary: Ref T3578. This is currently handled in a weird way in the Answer transaction. Instead, make it a Question transaction so, e.g., viewing Question transaction history shows who added answers and when.

Test Plan: Added answers to questions in Ponder.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3578

Differential Revision: https://secure.phabricator.com/D6603