Fix errors when mentioning others in Ponder

Summary: Fixes T9552. We need to set a questionID and the question object (for policy) when initializing a new Answer.

Test Plan: Write an answer that mentions another user.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9552

Differential Revision: https://secure.phabricator.com/D14263