R1:f1204c8c4568

Convert Ponder Questions to Ferret engine

Summary: See PHI177. Ref T12974. PonderQuestion was overlooked during the Ferret engine conversions.

Test Plan:
Ran migrations, searched for questions, got results:

{F5241185}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12974

Differential Revision: https://secure.phabricator.com/D18736