R1:ce16369f8e3f

Use Application PHIDs for ANSW

Summary:
Ref T2715. Ref T3578.

- Use Application PHIDs for Ponder Answers.
- Make Ponder answers policy-aware.
- Make PonderAnswerQuery policy-aware.

Test Plan: Used `phid.query`; browsed Ponder.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2715, T3578

Differential Revision: https://secure.phabricator.com/D6582