Clean up more PonderAnswer cruft

Summary: Ref T2715. Ref T3578. Moves PonderAnswerQuery toward being policy-aware so we can use application PHIDs.

Test Plan: Viewed various Ponder things, voted on answers.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2715, T3578

Differential Revision: https://secure.phabricator.com/D6581