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