Add a default moderation policy to Ponder

Summary: This allows installs to essentially set a "moderator" for Ponder, who can clean up answers. Fixes T9098

Test Plan: Edit an answer I don't own.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9098

Differential Revision: https://secure.phabricator.com/D13818