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