R1:2532cb961358

Add mail keys to Ponder questions

Summary:
We need to go slightly farther to stub reply handler functionality for Ponder in at least some configurations, where we rely on the presence of a unique random key to generate per-object or per-object+user reply addresses.

This should probably be formalized in an interface since it's currently pretty ad-hoc.

Test Plan:
- Made comments in Ponder under a per-user email configuration.
- Ran migration, verified mail keys were generated.
- Ran migration again…