Export Ponder question and answer bodies as remarkup blocks during transactions

Summary: Fixes T6189. We currently don't raise these to the editor level, so files, mentions, and project stuff get ignored.

Test Plan: Verified that files added to question and answer bodies end up attached to the relevant objects.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6189

Differential Revision: https://secure.phabricator.com/D10564